Set alpha as default

[JR-1991]

Use the alpha tag by default to prevent errors that may occur from the unstable tag.

Closes #11

Thanks to @pdurbin on Zulip - https://dataverse.zulipchat.com/#narrow/stream/375812-containers/topic/Dataverse.20Action.20broken/near/426731935

Edit @poikilotherm: we went with fixing the root cause instead - missing PID provider settings.

[pdurbin]

I see "Tag of image for Dataverse app and Configbaker" which sounds good. We want them both to change.

[pdurbin]

Heads up that "alpha" just shifted from 6.1 to 6.2 and 6.2 requires some new PID settings:

        -Ddataverse.pid.providers=fake
        -Ddataverse.pid.default-provider=fake
        -Ddataverse.pid.fake.type=FAKE
        -Ddataverse.pid.fake.label=FakeDOIProvider
        -Ddataverse.pid.fake.authority=10.5072
        -Ddataverse.pid.fake.shoulder=FK2/

Here's where I added them to the API test runner: gdcc/api-test-runner@a95a8e1