Publish feedback-http-server image in Dockerhub

As described in #5, this PR adds a GitHub workflow to build and publish a Docker image for feedback-http-server in DockerHub.

Code Review Checklist (to be filled out by reviewer)

[x] Description accurately reflects what changes are being made.
[x] Description explains why the changes are being made (or references an issue containing one).
[x] The PR appropriately sized.
[x] New code has enough tests.
[x] New code has enough documentation to answer "how do I use it?" and "what does it do?".
[x] Existing documentation is up-to-date, if impacted.