The run instruction will execute the command of your choosing while

building the image.

The command will be run exactly as if you had typed in a shell on the guest OS.

You can use these commands to permanently update the contents of the image.

Run is useful for installing dependencies or

running a script in the container at build time.

This pair of commands here both run the ubuntu apps get package manager.

The first updates the list of available packages and

the second uses that updated list to install python3.

The form of one instruction shown here is called the shell form which will execute

the given command using a shell like bash.

You should use this form if there are environment variables or

file name wild cards whose values you want substituted into the command.

The second form of RUN is called the exec form.

Let me convert this first instruction to exec form.

We use JSON array syntax with square brackets.

We'll surround each element with quotation marks and

we'll separate the elements with commas.

The first element should be the executable you want to run,

in this case that's apt-get.

All subsequent elements are arguments to that executable.

The exec form just passes your parameters directly to the given executable

without invoking a shell or doing any substitution.

You should use the exec form if you want to be very precise about the arguments you

pass to the executable.

So to finish converting this one, we'll use update as our second argument and

-y as our third.

And this run instruction in exec form is identical in meaning to this run

instruction in shell form.