Pushing docker image to project registry failed with 503 after it was successfully built

I have just deployed GitLab to Google Container Engine based on the demo video. Everything seems to be working fine until GitLab tries to push the docker image it has built for my project to the projects registry. Here what I get in the logs:

Removing intermediate container 4d6b87f6e65e
Step 8/8 : CMD http-server /usr/src/app/dist
 ---> Running in 76a36359ca09
time="2017-01-26T17:02:39.782174659Z" level=info msg="Layer sha256:0648264d81dbc8b6af1f5dad0f7e5c5dc3b000c488ca3bcba598beb244a19393 cleaned up" 
 ---> 2d485964d44d
Removing intermediate container 76a36359ca09
Successfully built 2d485964d44d
Logging to GitLab Container Registry with CI credentials...
Login Succeeded

Pushing to GitLab Container Registry...
The push refers to a repository [registry.dev.xxxxxx.com/xxxxxx/xxxxxx]
b9b0482dc33a: Preparing
7ab13542f934: Preparing
2a28ffb5f98f: Preparing
6385ba5d5991: Preparing
f984f4465482: Preparing
f8791c056b24: Preparing
2e08149e674a: Preparing
f55f65539fab: Preparing
30339f20ced0: Preparing
0eb22bfb707d: Preparing
a2ae92ffcd29: Preparing
f8791c056b24: Waiting
2e08149e674a: Waiting
f55f65539fab: Waiting
30339f20ced0: Waiting
0eb22bfb707d: Waiting
a2ae92ffcd29: Waiting
6385ba5d5991: Pushed
2a28ffb5f98f: Pushed
f8791c056b24: Pushed
2e08149e674a: Pushed
b9b0482dc33a: Pushed
f984f4465482: Pushed
0eb22bfb707d: Pushed
30339f20ced0: Retrying in 5 seconds
time="2017-01-26T17:03:31.550360229Z" level=error msg="Upload failed, retrying: received unexpected HTTP status: 503 Service Temporarily Unavailable" 
time="2017-01-26T17:03:32.393101165Z" level=error msg="Upload failed, retrying: received unexpected HTTP status: 503 Service Temporarily Unavailable" 
a2ae92ffcd29: Retrying in 5 seconds
30339f20ced0: Retrying in 4 seconds
[... same errors repeat ...]
7ab13542f934: Pushed
time="2017-01-26T17:06:30.384627182Z" level=error msg="Attempting next endpoint for push after error: received unexpected HTTP status: 503 Service Temporarily Unavailable" 
received unexpected HTTP status: 503 Service Temporarily Unavailable
time="2017-01-26T17:06:30.387319925Z" level=info msg="Processing signal 'terminated'" 
time="2017-01-26T17:06:30.391452418Z" level=info msg="stopping containerd after receiving terminated" 
ERROR: Build failed: error executing remote command: command terminated with non-zero exit code: Error executing in Docker Container: 1

I don't see any errors in /var/log/gitlab/registry/current

Any idea what may be broken or where I can look at for further details?

Thanks