Dockerfile: .git is required by Makefile

Author: ken-matsui

Diff for: Dockerfile

@@ -12,6 +12,7 @@ WORKDIR /app
 COPY .clang-format .
 COPY .clang-tidy .
 COPY poac.toml .
+COPY .git .
 COPY Makefile .
 COPY src ./src/