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.

.dockerignore

@@ -0,0 +1,21 @@
+# Build artifacts
+secret
+coverage.out
+*.test
+
+# IDE and editor files
+.vscode
+.idea
+*.swp
+*.swo
+*~
+
+# macOS
+.DS_Store
+
+# Claude files
+.claude/
+
+# Local settings
+.golangci.yml
+.claude/settings.local.json