Stable docker image tags

[joeyparrish]

The :latest Docker tag is the only Docker tag in the repo. Having the only tag always point to the latest commit has occasionally introduced some instability for me in dependent projects (e.g. #354). I propose that we add tags for release versions as well, automated through GitHub Actions workflows, and that you make a GitHub release soon after that.

@Stephane-D, what do you think?