Build and publish container images

[sajith]

Issue is #123. Here are the changes:

• A new GitHub Actions workflow has been added, which will build and publish container images tagged sdx-controller:latest and bapm-server:latest to Docker Hub. The images will appear under https://hub.docker.com/orgs/awsdx/repositories. This workflow will trigger when a commit is made to main, which will usually happen when a PR is merged.
• A major version of black was released in January, which makes some different formatting changes than the previous version, so some Python code that is unrelated to this PR has been re-formatted.
• Minor adjustments in "test" and "checks" workflows, for consistency's sake.

[coveralls]

Pull Request Test Coverage Report for Build 4188922796

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 42.24%

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Totals
Change from base Build 3894908329:	0.0%
Covered Lines:	577
Relevant Lines:	1366

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💛 - Coveralls