fix: docker build out-of-disk issue - aggressive cleanup

- .dockerignore: exclude build/cache/docs to reduce build context (59 lines)
- Dockerfile: add aggressive cleanup (cache, registry, git) after cargo build
- Dockerfile: use --locked flag for reproducible builds
- build.yaml: add pre-build disk cleanup (docker + cargo cache)
- build.yaml: clean cargo before Docker build to free space
- build.yaml: show disk usage before/after for debugging

Fixes 'No space left on device' error during Docker build in runner

.dockerignore

@@ -1,12 +1,55 @@
+# Git
 .git
 .gitignore
+.gitattributes
+
+# CI/CD
+.github
+.gitea
+.gitlab-ci.yml
+
+# Kubernetes
+k8s/
+helm/
+
+# Documentation
 *.md
-__pycache__
-*.pyc
-.env.local
-.venv
-venv/
-.pytest_cache
-.coverage
-htmlcov
+docs/
+
+# IDE
+.vscode
+.idea
+*.swp
+*.swo
+*~
+
+# OS
 .DS_Store
+Thumbs.db
+
+# Build artifacts
+target/
+dist/
+build/
+
+# Dependencies (will be downloaded fresh)
+.cargo/
+Cargo.lock.bak
+
+# Testing
+.coverage
+coverage/
+
+# Secrets
+.env
+.env.local
+.env.*.local
+
+# Archives
+*.tar
+*.tar.gz
+*.zip
+
+# Node (if any)
+node_modules/
+*.log