chore: enhance .dockerignore for Docker security

Author: nikolay-e

.dockerignore

@@ -31,7 +31,11 @@ venv/
 env/
 ENV/
 .venv
+
+# Environment files (all variants)
 .env
+.env.*
+!.env.example
 
 # IDEs
 .vscode/
@@ -52,6 +56,7 @@ htmlcov/
 .nox/
 .mypy_cache/
 .ruff_cache/
+.import_linter_cache/
 test-results/
 coverage.xml
 
@@ -120,3 +125,15 @@ mutants/
 # Node modules (will be built in Docker)
 gui/node_modules/
 gui/dist/
+
+# SSH keys (prevent accidental exposure)
+*id_rsa*
+*id_ed25519*
+*.pem
+*.key
+
+# Private directory
+private/
+
+# Secrets baseline
+.secrets.baseline