Support for local Docker builds

* add a git-ignored `local/` folder for Makefile/Dockerfile
* import `local/Makefile` to root Makefile
* add `.dockerignore`

.gitignore

@@ -44,7 +44,9 @@ docker-compose.yaml
 /conf/provisioning/**/custom.yaml
 profile.cov
 /grafana
+/local
 .notouch
+/Makefile.local
 /pkg/cmd/grafana-cli/grafana-cli
 /pkg/cmd/grafana-server/grafana-server
 /pkg/cmd/grafana-server/debug