login subcommand does not respect URL path, unlike registry which has url prefix setting.

Description

A docker registry using the distribution project can be configured with a url prefix instead of "/". this then makes it impossible to log in because the

Here's someone facing the same issue.

Steps to reproduce the issue:

1. start private registry with this config (http.prefix) set to /test, this can be done with a simple docker run command, as these variables are all also read from env vars.
2. run docker login localhost:5001/test and
3. notice that requests are coming on on localhost:5001/v2 and not localhost:5001/test/v2

Describe the results you received:

as described in #190, the url is simply cut off and then you can never reach the endpoint you need to continue.

Describe the results you expected:
I expected this software to behave like this one, which, for example, will request 2 different /.well-known urls before giving up. the one at the root, ./well-known, and then /the/path/given/as/argument/.well-known.

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

Output of docker version:

Docker version 19.03.12, build 48a66213fe

Output of docker info:

Client:
 Debug Mode: false

Server:
 Containers: 9
  Running: 1
  Paused: 0
  Stopped: 8
 Images: 29
 Server Version: 19.03.12
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc version: dc9208a3303feef5b3839f4323d9beb36df0a9dd
 init version: fec3683
 Security Options:
  apparmor
  seccomp
   Profile: default
 Kernel Version: 5.4.0-80-generic
 Operating System: Ubuntu 20.04.2 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 15.55GiB
 Name: tb1-Latitude-3440
 ID: VO74:Z6HH:U4JV:2B6T:YAWD:DVZA:Z3RX:SL37:WUA2:G7IT:O3BA:5BIW
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: No swap limit support

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

physical. (registry is running in the cloud, its proxied behind nginx, which is where i am seeing the urls it is requesting.

Start by reproducing the issue with a distribution registry configured with http.prefix set to /test, then run docker login localhost:5001/test and inspect the login subcommand's request construction. Done means requests use the supplied path, reaching localhost:5001/test/v2 rather than the root /v2 endpoint.