compatibility api should implement /auth endpoint

[troyready]

/kind feature

Description

The docker api /auth endpoint should be implemented.

Steps to reproduce the issue:

1. Post to /auth endpoint: curl -X POST -s --unix-socket $XDG_RUNTIME_DIR/podman/podman.sock http://localhost/auth

Describe the results you received:
{"cause":"","message":"Path /auth is not supported","response":0}

Additional information you deem important (e.g. issue happens only occasionally):
This will allow the use of the python SDK for docker login function, which uses this endpoint

Output of podman version:

Version:      3.0.0
API Version:  3.0.0
Go Version:   go1.15.8
Built:        Wed Dec 31 16:00:00 1969
OS/Arch:      linux/amd64

Output of podman info --debug:

host:
  arch: amd64
  buildahVersion: 1.19.4
  cgroupManager: systemd
  cgroupVersion: v2
  conmon:
    package: 'conmon: /usr/bin/conmon'
    path: /usr/bin/conmon
    version: 'conmon version 2.0.25, commit: unknown'
  cpus: 8
  distribution:
    distribution: debian
    version: unknown
  eventLogger: journald
  hostname: stargazer
  idMappings:
    gidmap:
    - container_id: 0
      host_id: 1000
      size: 1
    - container_id: 1
      host_id: 100000
      size: 65536
    uidmap:
    - container_id: 0
      host_id: 1000
      size: 1
    - container_id: 1
      host_id: 100000
      size: 65536
  kernel: 5.10.0-3-amd64
  linkmode: dynamic
  memFree: 29001703424
  memTotal: 50156310528
  ociRuntime:
    name: runc
    package: 'runc: /usr/bin/runc'
    path: /usr/bin/runc
    version: |-
      runc version 1.0.0~rc93+ds1
      commit: 1.0.0~rc93+ds1-2
      spec: 1.0.2-dev
      go: go1.15.8
      libseccomp: 2.5.1
  os: linux
  remoteSocket:
    exists: true
    path: /run/user/1000/podman/podman.sock
  security:
    apparmorEnabled: false
    capabilities: CAP_CHOWN,CAP_DAC_OVERRIDE,CAP_FOWNER,CAP_FSETID,CAP_KILL,CAP_NET_BIND_SERVICE,CAP_SETFCAP,CAP_SETGID,CAP_SETPCAP,CAP_SETUID,CAP_SYS_CHROOT
    rootless: true
    seccompEnabled: true
    selinuxEnabled: false
  slirp4netns:
    executable: /usr/bin/slirp4netns
    package: 'slirp4netns: /usr/bin/slirp4netns'
    version: |-
      slirp4netns version 1.0.1
      commit: 6a7b16babc95b6a3056b33fb45b74a6f62262dd4
      libslirp: 4.4.0
  swapFree: 0
  swapTotal: 0
  uptime: 11h 11m 56.37s (Approximately 0.46 days)
registries: {}
store:
  configFile: /home/troyready/.config/containers/storage.conf
  containerStore:
    number: 0
    paused: 0
    running: 0
    stopped: 0
  graphDriverName: overlay
  graphOptions:
    overlay.mount_program:
      Executable: /usr/bin/fuse-overlayfs
      Package: 'fuse-overlayfs: /usr/bin/fuse-overlayfs'
      Version: |-
        fusermount3 version: 3.10.2
        fuse-overlayfs: version 1.4
        FUSE library version 3.10.2
        using FUSE kernel interface version 7.31
  graphRoot: /home/troyready/.local/share/containers/storage
  graphStatus:
    Backing Filesystem: extfs
    Native Overlay Diff: "false"
    Supports d_type: "true"
    Using metacopy: "false"
  imageStore:
    number: 1
  runRoot: /run/user/1000/containers
  volumePath: /home/troyready/.local/share/containers/storage/volumes
version:
  APIVersion: 3.0.0
  Built: 0
  BuiltTime: Wed Dec 31 16:00:00 1969
  GitCommit: ""
  GoVersion: go1.15.8
  OsArch: linux/amd64
  Version: 3.0.0

Package info (e.g. output of rpm -q podman or apt list podman):

podman/testing,now 3.0.0+dfsg1-2 amd64 [installed]
podman/testing 3.0.0+dfsg1-2 i386