docker-build fails for DockerHub and Scaleway on PRs from forks

[rootulp]

Context

celestia-app just upgraded to v0.3.1 of this repo.

Problem

CI fails for docker-build to DockerHub and Scaleway with:

Error: Username and password required

Link: https://github.com/celestiaorg/celestia-app/actions/runs/7789514729/job/21241591600?pr=3071#step:3:11

Proposal

Skip this task if the workflow is expected to fail. I think it's expected to fail for PRs created from forks cc: @MSevey