Add Docker support for building and running the CLI tool

- Add DOCKER_HOST export to Makefile for remote Docker daemon
- Create multi-stage Dockerfile:
  - Build stage: golang:1.24-alpine with gcc, make, git
  - Runtime stage: alpine with ca-certificates, gnupg
  - Runs as non-root 'secret' user
- Add Makefile targets:
  - docker: build container as sneak/secret
  - docker-run: run container interactively
- Add .dockerignore to exclude build artifacts but keep .git
  for potential linker flags

Container includes GPG support for PGP unlockers and runs on Linux,
making it suitable for cross-platform testing and deployment.

Makefile

@@ -1,4 +1,5 @@
 export CGO_ENABLED=1
+export DOCKER_HOST := ssh://root@ber1app1.local
 
 default: check
 
@@ -23,6 +24,14 @@ lint:
 # Check all code quality (build + vet + lint + unit tests)
 check: ./secret vet lint test
 
+# Build Docker container
+docker: 
+	docker build -t sneak/secret .
+
+# Run Docker container interactively
+docker-run:
+	docker run --rm -it sneak/secret
+
 # Clean build artifacts
 clean:
 	rm -f ./secret