How can I copy `.git` directory to the container with GitHub actions?

Question

I have a GitHub actions CI which builds a my app in the last version of this app .git directory is mandatory for building it the problem is that GitHub action don't remove it in the docker build step of the CI.

Here is my CI template:

on:
  push:
  pull_request:

jobs:
  build:
    runs-on: ubuntu-20.04
    if: github.ref == 'refs/heads/3.3.5-MV'
    steps:
      - uses: actions/checkout@v2
      -
        name: Set up QEMU
        uses: docker/setup-qemu-action@v1
      -
        name: Set up Docker Buildx
        uses: docker/setup-buildx-action@v1
      -
        name: Login to DockerHub
        uses: docker/login-action@v1
        with:
          username: ${{ secrets.DOCKERHUB_USERNAME }}
          password: ${{ secrets.DOCKERHUB_TOKEN }}
      -
        name: Build and push
        id: docker_build
        uses: docker/build-push-action@v2
        with:
          file: ./Dockerfile
          tags: foo/bar:latest
          push: true
      -
        name: Image digest
        run: echo ${{ steps.docker_build.outputs.digest }}

The Dockerfile

FROM ubuntu:focal
ENV TZ=Europe/Paris
RUN ln -snf /usr/share/zoneinfo/$TZ /etc/localtime && echo $TZ > /etc/timezone
RUN apt update && apt upgrade -y && apt install -y [all my dependencies]

WORKDIR /usr/src/app

COPY . .

WORKDIR /usr/src/app/build

#default path on docker image /usr/local
RUN cmake ../ -DCMAKE_INSTALL_PREFIX=/usr/local/ -DTOOLS=0 

RUN make -j $(nproc) install

WORKDIR /usr/local/bin