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

@@ -65,4 +65,3 @@
 # The binary `make build` writes, and the CI workflow, which is not a
 # build input.
 /keyfunc
-.gitea