Add COUNTRY input validation and keep .git in Docker image

Validates COUNTRY env var is a 2-letter uppercase code. Keeps .git
directory in image since gemspec needs git ls-files at runtime.

Author: midwire

docker-entrypoint.sh

@@ -4,6 +4,10 @@ set -euo pipefail
 args=(--work-dir /output --generate-files --clobber)
 
 if [ -n "${COUNTRY:-}" ]; then
+  if [[ ! "$COUNTRY" =~ ^[A-Z]{2}$ ]]; then
+    echo "Error: COUNTRY must be a 2-letter uppercase code (e.g., US, AD)" >&2
+    exit 1
+  fi
   args+=(--country "$COUNTRY")
 fi