Add Coolify deployment support

Multi-stage Dockerfile builds the email-builder, admin frontend, and Go
backend from source and stuffs assets into the binary. The runtime-only
image moved to Dockerfile.goreleaser for release builds. Adds a Coolify
compose file with magic service variables, healthchecks, and volumes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.dockerignore

@@ -22,4 +22,6 @@
 **/secrets.dev.yaml
 **/values.dev.yaml
 LICENSE
-README.md
+README.mddocs
+dev
+.github