Fix failing nightly docker image build

[patcon]

The docker hub image build has been failing for a long time, and showing up as a failed badge on the README.

This should fix it.

[metasoarous]

Hey Pat; Thanks for doing this.

I'm not sure that we ever switched credentials over to the compdem account, so I may wait to verify that things are set up for that before merging, but otherwise this looks great.

Thanks again!

[metasoarous]

PS We may also want to switch from nightly to just triggering on push to dev (-> edge soon), but we can sort all that out in time.

[patcon]

Any loss to just merging this tiny fixup, and leaving the other fixup for later? workflow might still fail, but clearer error, and one less lingering PR :)