Work around `trusted_domains` not always being set

Sometimes when installing nextcloud, the trusted_domains setting is not populated from the NEXTCLOUD_TRUSTED_DOMAINS env var by the docker image's entrypoint. I mentioned this in an upstream issue here.

This causes Stackspin CI to fail from time to time. This could also happen during a regular (non-CI) install, though I haven't seen that yet. Perhaps #1052 was actually an instance of this.

We can try to "fix" this by setting trusted_domains explicitly in our config, not only through the env var.