Build on GCB during CI

[chadwhitacre]

We broke CI in sentry and snuba with #1251 and #1289, due to (not fully understood) discrepancies in the environment between local dev, self-hosted CI, and sentry/snuba CI. The proposal here is to trigger a GCB runner similar to the ones in sentry and snuba to run as part of self-hosted CI to ensure that we're not breaking CI over there with our changes here. This should cover us in the future even if we don't fully understand the root cause for the problem(s) with #1251 and #1289.

This is an action item for INC-98 (private).

[chadwhitacre]

There are triggers set up in GCB that are wired to these two build files:

• https://github.com/getsentry/sentry/blob/master/docker/cloudbuild.yaml
• https://github.com/getsentry/snuba/blob/master/cloudbuild.yaml

Those two files are similar but not identical. There are also files in other repos called cloudbuild.yaml. I don't know how much we want to DRY this up for this ticket, probably we should just do the minimum copy/paste to get the e2e tests going similar to sentry and snuba, which are identical at heart:

    args:
      - "-e"
      - "-c"
      - | 
        ./install.sh
        set +e
        ./test.sh
        test_return=$?
        set -e
        if [[ $test_return -ne 0 ]]; then
          echo "Test failed.";
          docker-compose ps;
          docker-compose logs;
          exit $test_return;
        fi