skeema / skeema / 6762477726

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docker: use tmpfs for truly ephemeral containers on Linux

This commit adjusts Docker logic to use tmpfs for the database data dir
in a few specific situations, where the container is removed soon after
creation anyway. This improves performance by avoiding disk writes. This
change is ignored outside of Linux, since Docker only supports tmpfs on an
actual Linux system (not Docker Desktop for Mac/Win, even though that uses
a Linux VM under the hood).

Impacted callsites include:

* workspace=docker combined with docker-cleanup=destroy
* applier subpackage integration test secondary container
* lower_case_table_names=1 integration test container

Future refactors to the integration test suite setup may permit this change
to be applied more broadly in CI environments. (Currently, containers are
stopped after each different subpackage's integration test run. On tmpfs, this
would mean each restart requires a DB re-init, greatly reducing the net
benefit to overall test performance.)

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