docker does not print custom 404 responses from the registry during login

[vikstrous]

Description

During push docker interprets 404 responses and allows a registry to return a custom message to the user, but during login it does not. It simply prints Error response from daemon: login attempt to http://172.17.0.1/v2/ failed with status: 404 Not Found

More specifically, during push the error message looks like:

error parsing HTTP 404 response body: invalid character 'C' looking for beginning of value: "Custom error information here\n"

That's good enough for us, but that could also be improved.

Steps to reproduce the issue:

1. create a docker registry with a custom 404 page that returns a hint as to why the registry is not accessible
2. try to docker login into the registry

Describe the results you received:

Error response from daemon: login attempt to http://172.17.0.1/v2/ failed with status: 404 Not Found

Describe the results you expected:

Error response from daemon: login attempt to http://172.17.0.1/v2/ failed with status: 404 Not Found and body: Please do XYZ to configure the registry before using it.

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

$ docker version
Client:
 Version:      1.12.2-cs2
 API version:  1.24
 Go version:   go1.6.3
 Git commit:   d5fda6e
 Built:        Sun Oct 23 07:48:43 2016
 OS/Arch:      linux/amd64

Server:
 Version:      1.13.0-dev
 API version:  1.25
 Go version:   go1.7.3
 Git commit:   17aaa08-unsupported
 Built:        Tue Nov  1 04:28:42 2016
 OS/Arch:      linux/amd64

Output of docker info:

$ docker info
Containers: 31
 Running: 17
 Paused: 0
 Stopped: 14
Images: 280
Server Version: 1.13.0-dev
Storage Driver: overlay2
 Backing Filesystem: extfs
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host macvlan null overlay
Swarm: active
 NodeID: q0i8qz897t67xphfahdlz2sa3
 Is Manager: true
 ClusterID: k33d8hpd40dri5jiukx2zgxze
 Managers: 1
 Nodes: 1
 Orchestration:
  Task History Retention Limit: 5
 Raft:
  Snapshot Interval: 10000
  Heartbeat Tick: 1
  Election Tick: 3
 Dispatcher:
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 3 months
  External CAs:
    cfssl: https://172.17.0.1:12381/api/v1/cfssl/sign
 Node Address: 172.17.0.1
Runtimes: runc
Default Runtime: runc
Security Options: apparmor seccomp
Kernel Version: 4.8.3
Operating System: NixOS 17.03.git.210b3b3 (Gorilla)
OSType: linux
Architecture: x86_64
CPUs: 4
Total Memory: 15.53 GiB
Name: thisisfine
ID: 4XB6:RAOL:2UT7:W3TT:JBSR:ILHC:LREA:HUVU:5U7V:TI7E:WUT2:WQTS
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): true
 File Descriptors: 203
 Goroutines: 346
 System Time: 2016-11-04T16:40:28.015262135-07:00
 EventsListeners: 2
Username: viktorstanchev
Registry: https://index.docker.io/v1/
WARNING: No cpu cfs quota support
WARNING: No cpu cfs period support
Insecure Registries:
 172.17.0.1
 127.0.0.0/8

Additional environment details (AWS, VirtualBox, physical, etc.):
Physical