feat: implement a simple docker file to run cli commands

[ohmayr]

This PR implements a minimalistic Dockerfile which runs the required CLI commands for Librarian.

Testing steps:

docker build . -t <image-name>

Command:

docker run --rm <image-name>

Output:

'generate' command executed.

Fixes: googleapis/librarian#899

[vchudnov-g]

Will you also have tests with dry_run=False?

I could imagine that when dry_run=False, you could accumulate the commands that would be yielded by dry_run=True and pass them to a function execute_run that will execute them. In that case, the tests for dry_run=False need merely verify that the invocations all call execute_run with the expected parameters, and then we only need a single additional test for execute_run that it executes a given test command.