GH Actions for automatic build and push image

[inistor]

Problem

An automated build is missing from openrepo; it is desired for two reasons:

• make sure that the build is still passing following branch changes, before merge
• have access to images built from branches, so developers can easily run tests with the changes from the branches
• streamlined release process (creating a release/tag automatically builds the image and pushes it to docker hub)

Approach

Create a Github workflow using Docker provided actions to compute the tags, based on branch names and releases, run the build and push to Docker Hub.

A mapping of how the image tags are generated based on git tags and branches can be found here.

The workflow will run for all events (push, tags) except pull_request.

❗ In order for the Docker Hub push to work, the following must be defined in the repo:

• A repository (⚠️ not environment) variable: DOCKERHUB_USERNAME. User for both login to Docker Hub and registry name for tagging
• A secret: DOCKERHUB_TOKEN - a token generated from Docker Hub with read/write access to the target Hub repository. Used for retrieving info about the registry (by the docker/metadata-action action and for pushing the built image - by docker/build-push-action

The image for this PR is ionnistor/openrepo:inistor-patch-docker-build