🐳 feat: implement Docker multi-stage build with caching optimization

Added Docker build infrastructure:
- Multi-stage build (builder, cache, production)
- Dependency hashing for cache invalidation
- GNU tar support for cache compatibility
- Production and development Dockerfiles
- Docker Compose for local development

Build Optimization:
- Dependency-based cache keys
- Layer caching strategy
- Cross-platform compatibility
- Gitea Actions cache integration

Files Added:
- docker/Dockerfile.build - Build environment
- docker/Dockerfile.prod - Production image
- docker/Dockerfile.prod.template - Template-based generation
- docker-compose.yml - Development setup
- scripts/calculate-deps-hash.sh - Cache key calculation
- scripts/test-docker-cache.sh - Cache testing

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

scripts/calculate-deps-hash.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+# Calculate dependency hash for Docker cache tag
+# This script calculates the hash used for the build cache image tag
+
+# Calculate hash of go.mod + go.sum
+# Use shasum on macOS, sha256sum on Linux
+if command -v sha256sum >/dev/null 2>&1; then
+    DEPS_HASH=$(sha256sum go.mod go.sum | sha256sum | cut -d' ' -f1 | head -c 12)
+else
+    DEPS_HASH=$(shasum -a 256 go.mod go.sum | shasum -a 256 | cut -d' ' -f1 | head -c 12)
+fi
+
+echo "Dependency hash: $DEPS_HASH"
+echo "$DEPS_HASH"
+
+# Export for use in other scripts
+if [ -n "$1" ]; then
+    echo "DEPS_HASH=$DEPS_HASH" > "$1"
+    echo "Exported to: $1"
+fi