CI: run on main, add a docker job, drop upstream-specific steps

The workflow only triggered on master. New job builds the image and runs
contrib/docker-smoke-test.sh, which checks the SSL handling of the entrypoint.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitignore

@@ -19,3 +19,6 @@
 /e_x.egg-info/
 
 .env
+
+.coverage
+/htmlcov/