feat(docker): Add mix deps.get to Dockerfile CMD

[joshlarson]

Summary of changes

Without this change, if a dependency is updated, then you have to fully rebuild the dotcom docker images. This change allows you to simply restart the containers, which will then pick up updated dependencies.

Depends on:

• refactor(docker): Unify dev Dockerfiles #2185
  • Dependency is to avoid git conflicts, since the Dockerfile(s) got moved in ☝️.

Note:

• Having mix deps.get and mix deps.compile in the Dockerfile as well is still helpful because having the dependencies cached in the image allows mix deps.get to run much more quickly, and won't need to rebuild the dependencies when restarting a container.

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General checks

• Are the changes organized into self-contained commits with descriptive and well-formatted commit messages? This is a good practice that can facilitate easier reviews.
• Testing. Do the changes include relevant passing updates to tests? This includes updating screenshots. Preferably tests are run locally to verify that there are no test failures created by these changes, before opening a PR.
• Tech debt. Have you checked for tech debt you can address in the area you're working in? This can be a good time to address small issues, or create Asana tickets for larger issues.

New UI, or substantial UI changes

• Cross-browser compatibility is less of an issue now that we're no longer supporting IE, but changes still need to work as expected in Safari, Chrome, and Firefox.
• Are interactive elements accessible? This includes at minimum having relevant keyboard interactions and visible focus, but can also include verification with screen reader testing.
• Other accessibility checks such as sufficient color constrast, or whether the layout holds up at 200% zoom level.

New endpoints, or non-trivial changes to current endpoints

• Have we load-tested any new pages or internal API endpoints that will receive significant traffic? See load testing docs
• If this change involves routes, does it work correctly with pertinent "unusual" routes such as the combined Green Line, Silver Line, Foxboro commuter rail, and single-direction bus routes like the 170?

[thecristen]

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