Add .dockerignore to reduce build context size and fix CI disk space error

The Docker build context was including .git directories (including submodules),
which added 500+ MB of unnecessary data. This caused 'no space left on device'
errors in GitHub Actions runners.

The .dockerignore file excludes:
- .git directories (saves ~500MB from revbayes submodule alone)
- Build artifacts (_build/, output/, *.o, *.a)
- Python cache files
- Documentation and IDE files
- CI/CD files

This reduces build context size and speeds up builds by avoiding copying
unnecessary files to Docker daemon.

Note: Git info for commit logging will not be available in container builds,
but this is acceptable for CI/production builds.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jgallowa07

.dockerignore

@@ -0,0 +1,41 @@
+# Git directories (including submodules) - can be hundreds of MB
+.git
+.gitignore
+.gitmodules
+
+# Build artifacts
+_build/
+output/
+*.o
+*.a
+*.so
+*.dylib
+
+# Python cache
+__pycache__/
+*.py[cod]
+*$py.class
+*.egg-info/
+.pytest_cache/
+
+# Documentation
+html/
+latex/
+Doxyfile
+
+# IDE and editor files
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+
+# CI/CD
+.github/
+
+# Docker files
+Dockerfile.base
+.dockerignore
+
+# Test outputs
+*.log