text: Title: failing docker tests locally on macOS (case 1)
  Body: <!-- Thank you for contributing. These HTML commments will not render in the issue, but you can delete them once you've read them if you prefer! -->
  
  ### Bug description
  <!-- Use this section to clearly and concisely describe the bug. -->
  With the latest versions of macOS (Catalina 10.15.4) and Docker CE (2.2.0.5), many of the tests fail to run locally.
  
  #### Expected behaviour
  <!-- Tell us what you thought would happen. -->
  Running the test suite locally on a released version of `repo2docker` should have no errors or failures.
  
  #### Actual behaviour
  <!-- Tell us what you actually happens. -->
  A number of tests which require running docker fail. The displayed message indicates Docker was unable to complete a volume mount:
  ```
  docker.errors.APIError: 500 Server Error: Internal Server Error ("b'Mounts denied: 
  The path /var/folders/fs/fjwjlc5x3yzgbw2nnnyzfcq80000gn/T/tmpxb7ynwyc
  is not shared from OS X and is not known to Docker.
  You can configure shared paths from Docker -> Preferences... -> File Sharing.
  See https://docs.docker.com/docker-for-mac/osxfs/#namespaces for more info.")
  ```
  Following the Docker instructions to add the path to the list of allowed mounting paths does not change the behavior.
  
  ### How to reproduce
  <!-- Use this section to describe the steps that a user would take to experience this bug. -->
  
  1. On a macOS box, checkout a released version of `repo2docker`
  2. Setup the development environment.
  3. invoke `pytest tests/unit/test_env.py` (as one example)
  4. observe failure.
  
  ### Your personal set up
  <!-- Tell us a little about the system you're using. You can see the guidelines for setting up and reporting this information at https://repo2docker.readthedocs.io/en/latest/contributing/contributing.html#setting-up-for-local-development. -->
  
   - OS: macOS
   - Docker version: CE 2.2.0.5
   - repo2docker version: 0.11.4
  
  ### Workaround
  
  While this is clearly a Docker problem, it is possible to tweak (some of) the tests to avoid needing to use the problematic directory paths.
  
  For example, in the `test_env.py` shown above, `repo2docker` is invoked from a directory that both:
  - is created in a manner that ensures the directory will be deleted at the end of the test (unless python itself crashes)
  - has no content
  
  Thus, changing the parent of the temp directory to be the git working directory allows the test to pass:
  ```diff
  -    with tempfile.TemporaryDirectory() as tmpdir:
  +    # There appear to be some odd combinations of default dir that do
  +    # not work on macOS Catalina with Docker CE 2.2.0.5, so use
  +    # the current dir -- it'll be deleted immediately
  +
  +    with tempfile.TemporaryDirectory(dir=os.path.abspath(os.curdir)) as tmpdir:
  ```
  
  This approach may not be applicable for all such locations, or desired for other reasons by the team. I'm happy to prepare a PR if project folks think this is a reasonable approach.