Add correct permissions for action

[EnriqueL8]

Due to new changes the GitHub Token in the action no longer has the write permissions and thus the actions fail when pushing the docker image to ghcr.io. To enable the GitHub token to have those permissions, it now needs to be specified within the job itself.

403 forbidden error https://github.com/hyperledger/firefly-signer/actions/runs/10359460639

The push refers to repository [ghcr.io/hyperledger/firefly-signer]
fecdbc4471ce: Preparing
31d7a001c31a: Preparing
96bc1885c3ff: Preparing
6f697f52d485: Preparing
denied: installation not allowed to Write organization package