inspect: display proxy config

[crazy-max]

related to docker/build-push-action#872 where I wanted to set proxy envs through docker config but could not see if it was set without this change: https://github.com/docker/build-push-action/actions/runs/5175036309/jobs/9322112341#step:6:136

  /usr/bin/docker buildx inspect --bootstrap --builder builder-bd6fd9e2-bff4-4452-89fc-c77bdad3af8c
  #1 [internal] booting buildkit
  #1 pulling image moby/buildkit:buildx-stable-1 0.1s done
  #1 creating container buildx_buildkit_builder-bd6fd9e2-bff4-4452-89fc-c77bdad3af8c0
  #1 creating container buildx_buildkit_builder-bd6fd9e2-bff4-4452-89fc-c77bdad3af8c0 0.5s done
  #1 DONE 0.7s
  Name:          builder-bd6fd9e2-bff4-4452-89fc-c77bdad3af8c
  Driver:        docker-container
  Last Activity: 2023-06-05 08:36:32 +0000 UTC
  
  Nodes:
  Name:           builder-bd6fd9e2-bff4-4452-89fc-c77bdad3af8c0
  Endpoint:       unix:///var/run/docker.sock
  Driver Options: image="moby/buildkit:buildx-stable-1"
  Status:         running
  Flags:          --debug
  Buildkit:       v0.11.6
  Platforms:      linux/amd64, linux/amd64/v2, linux/amd64/v3, linux/amd64/v4, linux/386
  Labels:
   org.mobyproject.buildkit.worker.executor:         oci
   org.mobyproject.buildkit.worker.hostname:         736e3a301e01
   org.mobyproject.buildkit.worker.network:          host
   org.mobyproject.buildkit.worker.oci.process-mode: sandbox
   org.mobyproject.buildkit.worker.selinux.enabled:  false
   org.mobyproject.buildkit.worker.snapshotter:      overlayfs
  Proxy config:
   HTTPS_PROXY: http://localhost:3128
   HTTP_PROXY:  http://localhost:3128
  GC Policy rule#0:
   All:           false
   Filters:       type==source.local,type==exec.cachemount,type==source.git.checkout
   Keep Duration: 48h0m0s
   Keep Bytes:    488.3MiB
  GC Policy rule#1:
   All:           false
   Keep Duration: 1440h0m0s
   Keep Bytes:    8.382GiB
  GC Policy rule#2:
   All:        false
   Keep Bytes: 8.382GiB
  GC Policy rule#3:
   All:        true
   Keep Bytes: 8.382GiB

[tonistiigi]

I get the usefulness of showing these values, but iiuc these are not builder/node specific but loaded from local docker CLI config. Therefore it is misleading to show them here (or at least should be in a separate global section). Another alternative would be to print it directly in actions.