title: Install python packages from github with Docker
body: <p>I am trying to install a package that is not on PyPi. i.e from github. Adding the repo as git+url to the requirements file gives</p>
  <pre><code>ERROR: Error [Errno 2] No such file or directory: 'git' while executing command git clone -q https://github.com/Rapptz/discord-ext-menus /tmp/pip-req-build-147rct22
  ERROR: Cannot find command 'git' - do you have 'git' installed and in your PATH?
  </code></pre>
  <p>Installing the packages is done with</p>
  <pre><code>RUN python3 -m pip install -r requirements.txt
  </code></pre>
  <p>as specified in the <a href="https://docs.docker.com/language/python/build-images/" rel="nofollow noreferrer">docs</a></p>
  <p>I also tried the solutions from <a href="https://stackoverflow.com/questions/55249689/pip-install-giturl-within-a-docker-environment">this</a>, but the answers mess up my other packages.</p>
  <p>The dockerfile is almost directly from the docs</p>
  <pre><code>FROM python:3.8-slim-buster
  
  WORKDIR /app
  
  COPY requirements.txt requirements.txt
  
  RUN python3 -m pip install -r requirements.txt
  
  COPY . .
  
  CMD [ &quot;python3&quot;, &quot;main.py&quot;]
  </code></pre>
  <p>requirements.txt</p>
  <pre><code>asyncpg==0.21.0
  git+https://github.com/Rapptz/discord-ext-menus
  discord.py==1.7.0
  pre-commit==2.10.1
  pyclean==2.0.0
  pylint==2.6.0
  python-dotenv==0.15.0
  </code></pre>