Allow developers to specify Docker image tags; reference them in Compose file

[matthew-li]

Description

**** Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change. ****

• Updated the Docker images to allow the user to specify a tag (defaulting to latest).
• Updated the script for building the images to optionally take a tag as input.
• Updated the Docker Compose file to reference specific tags:
  • If the IMAGE_TAG environment variable is set, images with that tag are used.
  • Otherwise, the latest images are used.
• Discussed these options in the Docker README.md.

This enables a developer to build a separate set of images (e.g., when testing a different branch) without overriding their existing images.

Type of change

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• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

**** Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration. ****

• Review the changes to README.md.
• Run build_images.sh both with and without an optional argument. Ensure that images are built and tagged successfully.
• Bring up the Docker Compose stack both with and without specifying an IMAGE_TAG environment variable. Ensure that the correct images are used.

PR Self Evaluation

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• My code follows the agreed upon best practices
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if needed)
• My changes generate no new warnings
• Any dependent changes have been merged and published in the appropriate modules
• I have performed a self-review of my own code