Send .gitea in the Docker build context (closes #63)

The development image copies .git into /src but left .gitea out, so in
the image git saw the CI workflow as deleted: `git status` was not
empty, `make build` stamped a -dirty version, and `git commit -a` would
have deleted the workflow. With the .gitea line gone from
.dockerignore, /src matches the commit it was built from.

Model: opus-5-5

.dockerignore

@@ -62,5 +62,6 @@
 **/.vscode
 **/*.sublime-*
 
-# The binary `make build` writes.
+# The binary `make build` writes, and the CI workflow, which is not a
+# build input.
 /keyfunc