Bump version to 7.1.2 & switch to gha ci for image deployments

[adamus1red]

The GHA workflow pulls the version numbers to build from the SOPEL_BRANCH environment variable in the Dockerfile rather than relying on changing it in 3 different places.

Has a matrix build for the different supported python versions with Python 3.9 defaulting to be the "main" release version as the latest tag.

Also fixes #33

Example run of GHA action: adamus1red/docker-sopel/actions/runs/1042705230

New built-time secrets

• IMAGE: name of the image to be tagged with (i.e. sopelirc/sopel)
• DOCKER_USERNAME: Username to login to docker hub with.
• DOCKER_PASSWORD: Access token to authenticate with docker hub.

[HumorBaby]

oh boy, merging from master?? 😰
A lot of this looks good, and since I'm so behind on switch to GHA, I imagine we can use almost everything here…
however, I would like to request the following before a real review:

• remove excess files from PR; I think your current branch has extra CI stuff, e.g., dependabot…??

then, I would like to discuss with you (and anyone else that wants to chime in) about:

• splitting out multi-arch from a first-step straight-forward build+label+push. This can keep the multiple python versions as you are doing now. Once this is merged in a nice and clean state, let's 100% get multi-arch done.

Gotta run, but wanted to share my thoughts without further delay. Let me know what you think

[adamus1red]

* splitting out multi-arch from a first-step straight-forward build+label+push. This can keep the multiple python versions as you are doing now. Once this is merged in a nice and clean state, let's 100% get multi-arch done.

@HumorBaby I've split the single build into #42 with single arch build. After it is done I'll add the multi-arch on top.

I would like to request the following before a real review:

* remove excess files from PR; I think your current branch has extra CI stuff, e.g., dependabot…??

I added Dependabot to keep the actions that are used by GHA upto date.