This diff makes several fixes:
- Rebase integration image on slim Debian variants (and install a few missing deps we still need, like ssh)
- Our integration image Dockerfile seems to have broken due to things upstream changing, fixed by this diff
- Fixes https://github.com/dagster-io/dagster/issues/1999
- Our snapshotted requirements.txt dependencies for Python 3 were previously based on a Python 3.5 environment, which was increasingly being left behind. This diff updates to use Python 3.7 for snapshotting a requirements.txt file, and adds PEP 496 environment markers to constrain installed dependencies s.t. the 3.5 and 3.8 image builds still succeed.
Final integration image build is here: https://buildkite.com/dagster/integration-image-builds/builds/46