Remove GOV_UK_APP_DOMAIN from Dockerfile

[thomasiles]

We used to include the GDS sso gem, which also depends on plek. This meant we had to set the GOV_UK_APP_DOMAIN env var to a dummy value in order to run the application in production.

We no longer depend on the GDS sso gem or plek, so we can remove this value from the Dockerfile.

See:
https://docs.publishing.service.gov.uk/repos/plek.html

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