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Building Images Using Dockerfiles
5:31Developers use configuration files to customize the tools they use every day. Likewise, Docker looks for a Dockerfile for instructions on how it should build an image. In this stage, we're going to learn about all the instructions you can include in a Dockerfile so that your image is configured just the way you want it.
Developers use configuration files to
customize the tools they use everyday.
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For example, shells look for
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config files so
they know what prompt to display for you.
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What functions to load and so on.
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Programming languages have tools that
use configuration files to build an app.
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Likewise, Docker looks for
a dockerfile for
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instructions on how it
should build an image.
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In this stage, we're going to learn about
all the instructions you can include in
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a dockerfile, so that your image is
configured just the way you want it.
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A dockerfile is just a plain text file
that you store in your application
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directory.
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Dockerfiles let you install application
dependencies within the image,
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specify how to run your app, set up
networking configuration, and much more.
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When you use the docker command-line
interface to build an image,
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it will look for
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your dockerfile, and use the instructions
it contains to set up your image.
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Lines of a dockerfile are either
comments or instructions.
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If a line has a pound or
hash symbol at the very beginning and
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only at the very beginning, it's treated
as a comment line and ignored by docker.
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All other lines are treated
as instructions.
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An instruction is several of
predefined key words followed by one or
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more arguments.
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By convention, an instruction
key word should be in all caps,
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although Docker itself
doesn't require that.
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You remember the image for our Python web
app that we created earlier in the course.
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We installed Python, copied a script from
the host file system into the images file
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system, and then set it up to run that
script when the container launched.
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Here's the dockerfile that we
used to create that image.
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If you want a copy,
see the teacher's notes.
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This file illustrates
several important commands,
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which we'll go over in
the next few videos.
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You can see FROM, RUN, ENTRYPOINT, CMD,
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WORKDIR, COPY, and
EXPOSE instructions here.
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To build an image based
on this dockerfile,
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we need to run the docker build
command from our terminal.
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We need to supply a repository name and
tag for
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the image,
which we'll do with the -t option.
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We'll use a repository
name of sample-web-app.
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Then we need to type a colon character
followed by the tag we want to apply.
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We'll use a tag of 1.0.
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Lastly we need to specify the directory
that contains our dockerfile.
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Since this is a Python web app I have it
stored in the directory called Python.
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If we run this we'll see docker carrying
out the instructions in our dockerfile
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one by one.
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It retrieves a copy of the Ubuntu image
which our image is going to be based on.
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It runs commands to install Python 3 and
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copies our app source code over from
the Python directory to the image.
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It sets our app up to run
when the container starts and
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it exposes network port 8080 which our
app will be listening for connection on.
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Now that we've created an image, we can
run it using the docker run command.
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This image exposes a port.
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But we'll need to publish that port
before anyone can connect to it.
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We'll publish the exposed port
8080 to port 8080 on the host,
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with the -p option, -p 8080:8080.
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Then we need to specify the image name and
tag we want to run.
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We'll use the same name and
tag we specified when
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running docker build, sample-web-app:1.0.
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When we hit enter, the container will run.
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If we make a connection to port 8080,
on our dot host with localhost:8080,
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it will be forwarded to the exposed
port 8080 on our container.
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We should get a response from our app,
which is running within the container.
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Our terminal is still attached to
the web app running on the container.
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So we can just go back to our terminal and
press Ctrl + C to shut it down.
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Since our web app is
the container's main process
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the container will shut down
as soon as the app does.
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Before we move on I want to show
you a couple possible variations of
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the command to build and tag images.
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Often, you'll be building an image
when you're already in the same
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directory that contains your dockerfile.
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If we were to change to
the Python directory and
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list files, our dockerfile would be there.
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If this is the case,
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you can just specify the current directory
as the one that contains your dockerfile.
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So let's build up the docker
build command again and
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edit it to work within
the current directory.
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The only thing we need to
change is the directory name,
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which we just changed to a single dot,
which specifies the current directory.
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You also don't have to specify
a particular tag when you're just
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experimenting with different builds.
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When you're running docker build and
you need a value to pass for
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the -t option,
you can specify just the repository name.
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So in this case,
that would be sample-web-app.
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And you can leave the tag
portion following the colon off.
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By default,
Docker will use a tag of latest.
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The same is true when running
a container with docker run.
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You'll still need to specify any
ports you want to publish, but
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you can just specify the repository
name and leave the tag off.
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Docker will use a default of latest.
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In the next few videos we'll take
a look at the instructions used in our
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dockerfile.
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See you there.
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