These are improvements for better supporting the requirements of other tests.
- Opted for passing an array reference instead of an ENV file. This seems to be a better approach and supports more than just ENV changes.
- Likewise, shifted to a `create` + `start` approach, instead of `docker run` for added flexibility.
- Using `TEST_TMP_CONFIG` instead of `PRIVATE_CONFIG` to make the difference in usage with config volume in tests more clear.
- Changed the config volume from read-only volume mount to be read-write instead, which seems required for other tests.
- Added notes about logged failures from a read-only config volume during container startup.
- Added `TEST_CA_CERT` as a default CA cert path for the test files volume. This can be used by default by openssl methods.