acceptance: acceptance tests should download prebuilt Docker images, not build them at test time

[rickystewart]

Last week the acceptance test TestComposeGSSPython started failing with the following error:

        python_1     | psql: error: private key file "/certs/client.root.key" must be owned by the current user or root

This is because the postgresql-client apt package was updated to pull in a new version of postgres, including this change. With #81313 I worked around this by manually pulling in an older version of postgresql-client (postgresql-client-11) that does not have this behavior. This is a temporary workaround that doesn't actually address the root cause of the issue, which is that building docker images at test time may result in a test environment that we have never tested before and which is not guaranteed to actually work.

Instead we should build these docker images once and reference them via tag, as is typically done with docker. This will ensure that our test environments don't suddenly change under us.

Jira issue: CRDB-15204

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