start tagging things?

[svenevs]

Once #112 lands we will not be able to build anything prior to 2.10 regardless of PANDOC_VERSION (since alpine is getting upgraded to 3.12 and test suite updates).

Probably the easiest thing to do is start tagging so we have a track record.

• 2.9.2.1: https://github.com/pandoc/dockerfiles/commit/9df2bb4fde15ed82b4198846d4d1e8aed67362d0 (#114 is a problem, we can always rewrite history to inject a commit before 2.10 (next commit))
• 2.10: https://github.com/pandoc/dockerfiles/commit/6f636fb0c080d1999a75907ddef08d598002a4da