build: fix Docker build and switch releases to the homelab Tekton pipeline

The Dockerfile was broken since the MCP endpoint was added: it built a
single file (cmd/domogeek/domogeek.go) instead of the whole package, so
mcp.go's symbols (calendarDayFor, newMCPHandler) were undefined. Fixes
that by building ./cmd/domogeek, pins the builder image to
golang:1.27-alpine (module now requires go 1.25+), and drops the
hardcoded GOOS/GOARCH=amd64 so multi-arch builds compile natively per
platform instead of always producing an amd64 binary.

Also fixes .Dockerignore (wrong case, never actually read by
docker/buildah) by renaming it to .dockerignore, and anchors its
"domogeek" pattern to /domogeek so it excludes only the compiled binary,
not the whole cmd/domogeek source directory (same bug as the .gitignore
fix in an earlier commit).

Removes build-docker.sh: image releases now go through the existing
homelab Tekton pipeline (gitea-docker-build-multiarch), triggered by a
Gitea webhook on tag push, same pattern as other repos (e.g.
dra-devices). Images move from docker.io/cyrilix/domogeek to
git.cyrilix.bzh/cyrilix/domogeek. Documented in CLAUDE.md and README.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Hu8wwahgecuCGfesTKCfEd

.dockerignore

@@ -0,0 +1,2 @@
+domogeek
+.git