Add support for environment variables with default value in docker-compose files

[andrcuns]

What are you trying to accomplish?

It is fairly common to parametrize image in docker-compose file via variable with default value. Currently environment variable in compose file is ignored completely.

This change allows to update image when it is defined as default value within environment variable.

Anything you want to highlight for special attention from reviewers?

Most of the logic is around adjusting the regexes, so that is probably where most attention is required.

How will you know you've accomplished your goal?

• Added unit tests for both file_parser and file_updater
• I also ran this change in an e2e scenario with my app that uses dependabot-core

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.