Print local credential usage after builds (#2068)

[njhale]

Print a message to stdout when local docker credentials are used during acorn build.
This should make it more obvious when builds fail due to invalid local
credentials.

The message is printed at the end of both successful and failed builds.

e.g. output of a failed build w/ local cred usage message

[+] Building 5.3s (11/11) FINISHED                                                                                                                                   
   // ... ELIDED BUILD OUTPUT                                                                                                              0.0s
 => ERROR exporting to image                                                                                                                                    3.9s
 => => exporting layers                                                                                                                                         0.0s
 => => exporting manifest sha256:db534d916562a0663ee07bd52e8549431d6b13cc8c32c26e1a9ca55fdde61e28                                                               0.0s
 => => exporting config sha256:e4081843971dd0852fa32d8e83a21d5842887e826a1c252d0b1fd1892302c385                                                                 0.0s
 => => pushing layers                                                                                                                                           3.9s
------
 > exporting to image:
------
Used local credentials for: ghcr.io, index.docker.io
  ✗  ERROR:  failed to solve: failed to push ghcr.io/njhale/local-private-acornacorn: failed to authorize: failed to fetch oauth token: unexpected status: 403 Forbidden

Addresses #2068