Refactor PullRequest Image Building and Testing GitHub Actions CI

[scottyhq]

This PR also re-configures the Pull Request GitHub Action workflow to pass the base-notebook image between jobs as an "artifact". This is avoids the need to authenticate with docker hub, which is inconvenient for secrets management - PRs often come from forked repositories, but those workflows DO NOT have access to secrets stored in this repository (https://github.51.almunity/t5/GitHub-Actions/Make-secrets-available-to-builds-of-forks/td-p/30678/page/2 ).

This pull request supersedes:
#120
#121
#123

[scottyhq]

seeing a lot of UnsatisfiableError: The following specifications were found to be incompatible with the existing python installation in your environment: in the build logs for pangeo-notebook and pangeo-esip.

  - cmocean -> python[version='2.7.*|3.4.*|3.5.*|3.6.*']
  - eofs -> python[version='2.7.*|3.4.*|3.5.*|3.6.*']
  - palettable -> python[version='2.7.*|3.4.*|3.5.*|3.6.*']
  - watermark -> python[version='2.7.*|3.4.*|3.5.*|3.6.*|>=2.7,<2.8.0a0|>=3.5,<3.6.0a0|>=3.6,<3.7.0a0']
  - xrft -> python[version='2.7.*|3.5.*|3.6.*']

Your python: python=3.7

  - eofs -> python[version='2.7.*|3.4.*|3.5.*|3.6.*']
  - geolinks -> python[version='2.7.*|3.4.*|3.5.*|3.6.*']
  - pocean-core -> python[version='2.7.*|3.5.*|3.6.*']
  - pyoos -> python[version='2.7.*|3.4.*|3.5.*|3.6.*']
  - utide -> python[version='2.7.*|3.4.*|3.5.*|3.6.*']
  - xrft -> python[version='2.7.*|3.5.*|3.6.*']

Eventually leading to failed conda installs with

Traceback (most recent call last):
  File "/usr/local/bin/r2d_overlay.py", line 167, in <module>
    main()
  File "/usr/local/bin/r2d_overlay.py", line 161, in main
    build()
  File "/usr/local/bin/r2d_overlay.py", line 122, in build
    ['/bin/bash', '-c', command], preexec_fn=applicator._pre_exec
  File "/srv/conda/envs/notebook/lib/python3.7/subprocess.py", line 363, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['/bin/bash', '-c', 'conda env update -p /srv/conda/envs/notebook -f /home/jovyan/.onbuild-child/binder/environment.yml']' returned non-zero exit status 1.
The command '/bin/sh -c /usr/local/bin/r2d_overlay.py build' returned a non-zero code: 1

This was possibly due to a release of Repo2Docker today, so I'm pinning to the previous version 0.10 to reduce the number of changes https://github.com/jupyter/repo2docker/releases/tag/0.11.0

[scottyhq]

To speed up building I've removed the ML-related images from pangeo-notebook. I'm thinking we keep those in a separate ML-focused notebook #119

[scottyhq]

Came across a failed build due to conda network error:

2020-02-05T20:34:59.5638308Z Step 2/2 : FROM pangeo/base-notebook-onbuild:$VERSION
2020-02-05T20:34:59.5643353Z # Executing 5 build triggers
2020-02-05T20:35:11.0651653Z  ---> Running in fb90ae00402e
2020-02-05T20:35:38.1972491Z Removing intermediate container fb90ae00402e
2020-02-05T20:35:38.3746330Z  ---> Running in d038294a9c4a
2020-02-05T20:36:40.4058508Z Collecting package metadata (repodata.json): ...working... failed
2020-02-05T20:36:40.4064572Z �[91m
2020-02-05T20:36:40.4065012Z CondaHTTPError: HTTP 000 CONNECTION FAILED for url <https://conda.anaconda.org/conda-forge/linux-64/repodata.json>
2020-02-05T20:36:40.4065302Z Elapsed: -
2020-02-05T20:36:40.4065361Z 
2020-02-05T20:36:40.4065475Z An HTTP error occurred when trying to retrieve this URL.
2020-02-05T20:36:40.4065598Z HTTP errors are often intermittent, and a simple retry will get you on your way.
2020-02-05T20:36:40.4066267Z ConnectionError(ReadTimeoutError("HTTPSConnectionPool(host='conda.anaconda.org', port=443): Read timed out."))
2020-02-05T20:36:40.4066347Z 
2020-02-05T20:36:40.4066415Z 
2020-02-05T20:36:40.7354874Z �[0m�[91mTraceback (most recent call last):
2020-02-05T20:36:40.7356440Z   File "/usr/local/bin/r2d_overlay.py", line 147, in <module>
2020-02-05T20:36:40.7356977Z     main()
2020-02-05T20:36:40.7357176Z   File "/usr/local/bin/r2d_overlay.py", line 141, in main
2020-02-05T20:36:40.7357308Z     build()
2020-02-05T20:36:40.7357456Z   File "/usr/local/bin/r2d_overlay.py", line 109, in build
2020-02-05T20:36:40.7357891Z     ['/bin/bash', '-c', command], preexec_fn=applicator._pre_exec
2020-02-05T20:36:40.7358052Z   File "/srv/conda/envs/notebook/lib/python3.7/subprocess.py", line 347, in check_call
2020-02-05T20:36:40.7358220Z     raise CalledProcessError(retcode, cmd)
2020-02-05T20:36:40.7358661Z subprocess.CalledProcessError: Command '['/bin/bash', '-c', 'conda env update -p /srv/conda/envs/notebook -f /home/jovyan/.onbuild-child/binder/environment.yml']' returned non-zero exit status 1.
2020-02-05T20:36:40.8872912Z The command '/bin/sh -c /usr/local/bin/r2d_overlay.py build' returned a non-zero code: 1