Use of `--pre` in `requirements.txt` breaks things in strange ways

[gforsyth]

I was just trying to cut a new release of xonsh and ran into this -- took me a while to track it down.

Someone added a --pre specifier to our requirements.txt, which, on any rever activity that requires building a docker container fails now with

EnvironmentLocationNotFound: Not a conda environment: /root/black

The command '/bin/sh -c wget --quiet https://repo.anaconda.com/miniconda/Miniconda3-latest-Linux-x86_64.sh -O ~/miniconda.sh &&     /bin/bash ~/miniconda.sh -b -p /opt/conda &&     rm ~/miniconda.sh &&     /opt/conda/bin/conda clean -tipsy &&     ln -s /opt/conda/etc/profile.d/conda.sh /etc/profile.d/conda.sh &&     echo ". /opt/conda/etc/profile.d/conda.sh" >> ~/.bashrc &&     echo "conda activate base" >> ~/.bashrc &&     conda config --set always_yes yes &&     conda config --add channels conda-forge &&     conda update --all &&     conda install         --pre black cloud_sptheme codecov coverage doctr flake8         flake8-docstrings livereload matplotlib mypy numpy numpydoc         pip pre-commit prompt_toolkit psutil py pygments pytest         pytest-cov pytest-timeout pyzmq runthis-sphinxext sphinx         tornado &&     conda clean --all &&     conda info' returned a non-zero code: 1
activity-error:pytest:activity failed with execption:
Traceback (most recent call last):
  File "/opt/miniconda3/envs/rever/lib/python3.8/site-packages/rever/activity.xsh", line 83, in __call__
    self.func(*args, **kwargs)
  File "/opt/miniconda3/envs/rever/lib/python3.8/site-packages/rever/activities/pytest.xsh", line 27, in _func
    super()._func(code=command)
  File "/opt/miniconda3/envs/rever/lib/python3.8/site-packages/rever/activity.xsh", line 401, in _func
    docker.ensure_images(force_base=force_base, force_install=force_install)
  File "/opt/miniconda3/envs/rever/lib/python3.8/site-packages/rever/docker.xsh", line 284, in ensure_images
    build_image(base_file, base_image, make_base_dockerfile, **base_kwargs)
  File "/opt/miniconda3/envs/rever/lib/python3.8/site-packages/rever/docker.xsh", line 262, in build_image
    ![docker build -t @(image) -f @(dockerfile) --no-cache .]
  File "/opt/miniconda3/envs/rever/lib/python3.8/site-packages/xonsh/__amalgam__.py", line 20791, in subproc_captured_hiddenobject
    return xonsh.procs.specs.run_subproc(cmds, captured="hiddenobject", envs=envs)
  File "/opt/miniconda3/envs/rever/lib/python3.8/site-packages/xonsh/procs/__amalgam__.py", line 3494, in run_subproc
    command.end()
  File "/opt/miniconda3/envs/rever/lib/python3.8/site-packages/xonsh/procs/__amalgam__.py", line 907, in end
    self._end(tee_output=tee_output)
  File "/opt/miniconda3/envs/rever/lib/python3.8/site-packages/xonsh/procs/__amalgam__.py", line 926, in _end
    self._raise_subproc_error()
  File "/opt/miniconda3/envs/rever/lib/python3.8/site-packages/xonsh/procs/__amalgam__.py", line 1052, in _raise_subproc_error
    raise subprocess.CalledProcessError(rtn, spec.args, output=self.output)
subprocess.CalledProcessError: Command '['docker', 'build', '-t', 'xonsh/rever-base', '-f', 'rever/rever-base.dockerfile', '--no-cache', '.']' returned non-zero exit status 1.
rewinding to d3eda8e90bafe4edfc712c1038dec4b8267280e0

The --pre survives into the rever-base.dockerfile and ends up as

RUN wget --quiet https://repo.anaconda.com/miniconda/Miniconda3-latest-Linux-x86_64.sh -O ~/miniconda.sh && \
    /bin/bash ~/miniconda.sh -b -p /opt/conda && \
    rm ~/miniconda.sh && \
    /opt/conda/bin/conda clean -tipsy && \
    ln -s /opt/conda/etc/profile.d/conda.sh /etc/profile.d/conda.sh && \
    echo ". /opt/conda/etc/profile.d/conda.sh" >> ~/.bashrc && \
    echo "conda activate base" >> ~/.bashrc && \
    conda config --set always_yes yes && \
    conda config --add channels conda-forge && \
    conda update --all && \
    conda install \
        --pre black cloud_sptheme codecov coverage doctr flake8 \
        flake8-docstrings livereload matplotlib mypy numpy numpydoc \
        pip pre-commit prompt_toolkit psutil py pygments pytest \
        pytest-cov pytest-timeout pyzmq runthis-sphinxext sphinx \
        tornado && \
    conda clean --all && \
    conda info

Note the --pre in the conda install line. Clearly this does something weird that then breaks the build.

Workaround is to remove --pre.