Add docker-compose, RUNBOOK.md, and docker Makefile target

- docker-compose.yml: single service with data volume, ports 8443/9443/8080
- RUNBOOK.md: health checks, common operations (start/stop/backup/init),
  FIDO2 key registration, incident procedures (won't start, DB corruption,
  cert expiry, disk full, sync failures), escalation path
- Makefile: added docker target

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

Makefile

@@ -1,4 +1,4 @@
-.PHONY: build test test-race vet lint proto proto-lint clean all
+.PHONY: build test test-race vet lint proto proto-lint clean docker all
 
 LDFLAGS := -trimpath -ldflags="-s -w -X main.version=$(shell git describe --tags --always --dirty 2>/dev/null || echo dev)"
 
@@ -32,4 +32,7 @@ proto-lint:
 clean:
 	rm -f eng-pad-server
 
+docker:
+	docker build -t eng-pad-server -f Dockerfile .
+
 all: vet lint test eng-pad-server