Adjust Docker Hub build action to include PRs

[bachya]

Describe what the PR does:

This PR updates our Docker Hub workflow to publish images of PRs. This will help in a couple of ways:

1. It'll allow us to update/triage/diagnose Docker issues within a PR (vs. going straight to dev).
2. For longer-running PRs, it'll allow Docker users to test the PR changes easily.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #290 (bf8e9e3) into dev (eb076ec) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               dev      #290   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         1132      1132           
=========================================
  Hits          1132      1132           

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[bachya]

Confirmed that the push worked as expected: