pack builder ignores DOCKER_HOST

[radriaanse]

Describe the bug

When using the pack builder the DOCKER_HOST environment variable doesn't seem to be respected.
This causes the build to fail when the Docker socket is on a different location as default /var/run/docker, for example when running in rootless mode.

Interestingly the variable is properly used when creating the initial build pack container

waypoint/builtin/pack/builder.go

Line 78 in b550e5c

client.FromEnv,

When I capture the analyzer container using docker inspect it shows the socket mount to the wrong path (abbreviated output);

{
  "Id": "5baa0c8737efe0045d39d3f41b69dc5aed72204a383cf243fbdffb7c16d5746c",
  "Created": "2021-05-04T12:56:29.277985308Z",
  "Path": "/cnb/lifecycle/analyzer",
  "Args": [
    "-daemon",
    "-cache-dir",
    "/cache",
    "-layers",
    "/layers",
    "index.docker.io/library/go:latest"
  ],
  "HostConfig": {
    "Binds": [
      "/var/run/docker.sock:/var/run/docker.sock",
      "pack-cache-fec58b24b18f.build:/cache",
      "pack-layers-htmzbpxzfb:/layers",
      "pack-app-mekfvmheeo:/workspace"
    ]
  },
  "Mounts": [
    {
      "Type": "bind",
      "Source": "/var/run/docker.sock",
      "Destination": "/var/run/docker.sock",
      "Mode": "",
      "RW": true,
      "Propagation": "rprivate"
    }
  ],
  "Config": {
    "Env": [
      "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
      "CNB_USER_ID=1000",
      "CNB_GROUP_ID=1000",
      "STACK=heroku-18",
      "CNB_STACK_ID=heroku-18"
    ],
    "Cmd": [
      "/cnb/lifecycle/analyzer",
      "-daemon",
      "-cache-dir",
      "/cache",
      "-layers",
      "/layers",
      "index.docker.io/library/go:latest"
    ],
    "Image": "pack.local/builder/7474787872686a736b68:latest",
    "Volumes": null
  }
}

Steps to Reproduce

> echo $DOCKER_HOST
unix:///run/user/1000/docker.sock

project = "testing"

app "go" {
    build {
        use "pack" {}
    }

    deploy {
        use "docker" {}
    }
}

> waypoint up -v
...
 │ 2021/05/04 14:48:31.390228 INFO:   ===> DETECTING
 │ [detector] ======== Output: heroku/ruby@0.0.1 ========
 │ [detector] no
 │ [detector] err:  heroku/ruby@0.0.1 (1)
 │ [detector] heroku/go       0.3.1
 │ [detector] heroku/procfile 0.6.2
 │ 2021/05/04 14:48:31.913333 INFO:   ===> ANALYZING
 │ [analyzer] ERROR: failed to initialize docker client: failed to connect to docke
 │ r socket: dial unix /var/run/docker.sock: connect: connection refused
! failed with status code: 1
2021-05-04T14:48:32.312+0200 [WARN]  waypoint: context cancelled, stopping interrupt listener loop

Expected behavior
Would it be possible for Waypoint to inform CNB about the different location?

Waypoint Platform Versions
Additional version and platform information to help triage the issue if
applicable:

• Waypoint CLI Version: v0.3.1 (b550e5ce)
• Waypoint Server Platform and Version: v0.3.1
• Waypoint Plugin: pack & docker