Add Dockerfile and docker-master build target

Two-stage build: golang:1.25-alpine builder, alpine:3.21 runtime.
Produces a minimal container image for mcp-master.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Makefile

@@ -32,6 +32,11 @@ proto-lint:
 	buf lint
 	buf breaking --against '.git#branch=master,subdir=proto'
 
+docker-master:
+	podman build -f Dockerfile.master \
+		--build-arg VERSION=$(shell git describe --tags --always --dirty) \
+		-t mcr.svc.mcp.metacircular.net:8443/mcp-master:$(shell git describe --tags --always --dirty) .
+
 clean:
 	rm -f mcp mcp-agent mcp-master