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. The comment
above /keyfunc now names only the binary, as .gitignore does.

Model: opus-5-5

.dockerignore

@@ -62,7 +62,5 @@
 **/.vscode
 **/*.sublime-*
 
-# The binary `make build` writes, and the CI workflow, which is not a
-# build input.
+# The binary `make build` writes.
 /keyfunc
-.gitea