fix exit code when manifest merge failed

[jaihwan104]

When the immutable policy is applied to tags and the manifest push fails, the exit code is calculated as 0. This issue occurs when using the remote driver.

We are building images in GitLab CI. Even though the image was not pushed properly, the exitCode is calculated as 0, so the job is not stopping properly.

fix exit code when manifest merge failed as follow:

before:

bash-5.2# docker buildx build --push --platform linux/amd64,linux/arm64 -f Dockerfile -t myregistry.com/hello/world:test .
...
 => => pushing layers                                                                                                                                                         0.8s
 => => pushing manifest for myregistry.com/hello/world                                                                                                                  0.9s
 => [auth] hello/world:pull,push token for myregistry.com                                                                                               0.0s
 => [auth] hello/world:pull,push token for myregistry.com                                                                                                  0.0s
 => ERROR merging manifest list myregistry.com/hello/world:test                                                                                                         0.3s
------
 > merging manifest list myregistry.com/hello/world:test:
------

bash-5.2# echo $?
0

after:

bash-5.2# docker buildx build --push --platform linux/amd64,linux/arm64 -f Dockerfile -t myregistry.com/hello/world:test .
...
 => => pushing layers                                                                                                                                                         1.0s
 => => pushing manifest for myregistry.com/hello/world                                                                                                                  1.2s
 => [auth] hello/world:pull,push token for myregistry.com                                                                                                               0.0s
 => [auth] hello/world:pull,push token for myregistry.com                                                                                                               0.0s
 => ERROR merging manifest list myregistry.com/hello/world:test                                                                                                         1.0s
------
 > merging manifest list myregistry.com/hello/world:test:
------
ERROR: failed commit on ref "index-sha256:949e4827d78c5cb6e2e6445804681208db7ed53dfb6417081b342f7faceceb37": unexpected status from PUT request to https://myregistry.com/v2/hello/world/manifests/test: 412 Precondition Failed

bash-5.2# echo $?
1

[jaihwan104]

@tonistiigi Thank you for the review. I'd like to apply the official version to the production. Could you please add it to milestone v0.14.1?