bake: allow BAKE_CMD_CONTEXT builtin var

[tonistiigi]

fix #641

Allows accessing the main context for bake command from bake
file that has been imported remotely.

# docker buildx bake "git://github.com/docker/cli" --print
{
  "target": {
    "binary": {
      "context": "git:/github.com/docker/cli",
      "dockerfile": "Dockerfile",
      "args": {
        "BASE_VARIANT": "alpine",
        "GO_STRIP": "",
        "VERSION": ""
      },
      "target": "binary",
      "platforms": [
        "local"
      ],
      "output": [
        "build"
      ]
    }
  }
}

/tmp/a # touch foo bar
/tmp/a # docker buildx bake "git://github.com/tonistiigi/buildx#remote-test"
...
 > [4/4] RUN ls -l && stop:
#9 0.207 total 0
#9 0.207 -rw-r--r--    1 root     root             0 Jul 13 03:39 bar
#9 0.207 -rw-r--r--    1 root     root             0 Jul 13 03:39 foo
#9 0.207 /bin/sh: stop: not found


 # docker buildx bake "git://github.com/tonistiigi/buildx#remote-test" "git://github.com/docker/cli"
...
#8 0.232 -rw-r--r--    1 root     root          1893 Jul 13 03:58 poule.yml
#8 0.232 drwxr-xr-x    7 root     root          4096 Jul 13 03:58 scripts
#8 0.232 drwxr-xr-x    3 root     root          4096 Jul 13 03:58 service
#8 0.232 drwxr-xr-x    2 root     root          4096 Jul 13 03:58 templates
#8 0.232 drwxr-xr-x   11 root     root          4096 Jul 13 03:58 vendor
#8 0.232 -rwxr-xr-x    1 root     root         10123 Jul 13 03:58 vendor.conf
#8 0.232 /bin/sh: stop: not found

#   docker buildx bake "git://github.com/tonistiigi/buildx#remote-test" "https://ftp.gnu.org/gnu/binutils/binutils-2.36.tar.gz"
...
 > [4/4] RUN ls -l && stop:
#8 0.250 total 4
#8 0.251 drwxrwxrwx   19 root     root          4096 Jul 13 04:00 binutils-2.36
#8 0.251 /bin/sh: stop: not found

Signed-off-by: Tonis Tiigi tonistiigi@gmail.com

[tonistiigi]

This also fixed the issue where remote URLs were used in bake targets. Previously the values always passed path.Clean that corrupted the url.

[crazy-max]

@tonistiigi LGTM. In a follow-up, we could reserve variables starting with BAKE_ as we discussed.

[crazy-max]

Document cwd protocol

[tonistiigi]

I'm not sure if it is a good idea. I don't want users to write it in the bake file. If they do then they break the remote URL case. They should use the variable and this should be the internal behavior of the variable. We should be able to change that to workdir:// for example and no old files should be broken.

We should document the builtin var, and also more docs for vars/attrs in general.

[crazy-max]

they break the remote URL

Ah yes right

We should document the builtin var, and also more docs for vars/attrs in general.

👍