Optionally trigger integration tests

[josephmyers]

We don't always want to run the integrations tests, which cause the test run to take 7 minutes. Trigger integration tests (by default) only on the build server, not on dev machines.

RUN_UNIT_TESTS in the Dockerfile is likely what controls whether or not the tests run.

(Not required for this is providing an option to run the integration tests locally, assuming "dotnet test" does so for us. That would be a separate task.)