Migrate CI/deploy pipeline from GitHub Actions to Gitea Actions

Build/test/push now runs as .gitea/workflows/docker-build-push.yml,
pushing to gitea.blairhaus.net/pmb/finger and auto-deploying to mammut
and bsd on every push to main, since GitHub is no longer in use.
Removes the now-dead .github workflows, .codecov.yml, and their README
badges.

README.md

@@ -1,8 +1,5 @@
 # finger
 
-[![CI](https://github.com/waffle2k/finger/workflows/CI/badge.svg)](https://github.com/waffle2k/finger/actions)
-[![codecov](https://codecov.io/gh/waffle2k/finger/branch/main/graph/badge.svg)](https://codecov.io/gh/waffle2k/finger)
-
 A silly finger service written in c++20
 
 # Compiling: