Annotate flags that are not supported by Buildkit

[thaJeztah]

Before this patch:

Usage:	docker build [OPTIONS] PATH | URL | -

Build an image from a Dockerfile

Options:
      --add-host list           Add a custom host-to-IP mapping (host:ip)
      --build-arg list          Set build-time variables
      --cache-from strings      Images to consider as cache sources
      --cgroup-parent string    Optional parent cgroup for the container
      --cpu-period int          Limit the CPU CFS (Completely Fair Scheduler) period
      --cpu-quota int           Limit the CPU CFS (Completely Fair Scheduler) quota
  -c, --cpu-shares int          CPU shares (relative weight)
      --cpuset-cpus string      CPUs in which to allow execution (0-3, 0,1)
      --cpuset-mems string      MEMs in which to allow execution (0-3, 0,1)
      --disable-content-trust   Skip image verification (default true)
  -f, --file string             Name of the Dockerfile (Default is 'PATH/Dockerfile')
      --force-rm                Always remove intermediate containers
      --iidfile string          Write the image ID to the file
      --isolation string        Container isolation technology
      --label list              Set metadata for an image
  -m, --memory bytes            Memory limit
      --memory-swap bytes       Swap limit equal to memory plus swap: '-1' to enable unlimited swap
      --network string          Set the networking mode for the RUN instructions during build (default "default")
      --no-cache                Do not use cache when building the image
  -o, --output stringArray      Output destination (format: type=local,dest=path)
      --platform string         Set platform if server is multi-platform capable
      --progress string         Set type of progress output (auto, plain, tty). Use plain to show container output (default "auto")
      --pull                    Always attempt to pull a newer version of the image
  -q, --quiet                   Suppress the build output and print image ID on success
      --rm                      Remove intermediate containers after a successful build (default true)
      --secret stringArray      Secret file to expose to the build (only if BuildKit enabled): id=mysecret,src=/local/secret
      --security-opt strings    Security options
      --shm-size bytes          Size of /dev/shm
      --squash                  Squash newly built layers into a single new layer
      --ssh stringArray         SSH agent socket or keys to expose to the build (only if BuildKit enabled) (format: default|<id>[=<socket>|<key>[,<key>]])
  -t, --tag list                Name and optionally a tag in the 'name:tag' format
      --target string           Set the target build stage to build.
      --ulimit ulimit           Ulimit options (default [])

With this patch applied:

DOCKER_BUILDKIT=1 docker build --help

Usage:	docker build [OPTIONS] PATH | URL | -

Build an image from a Dockerfile

Options:
      --add-host list           Add a custom host-to-IP mapping (host:ip)
      --build-arg list          Set build-time variables
      --cache-from strings      Images to consider as cache sources
      --disable-content-trust   Skip image verification (default true)
  -f, --file string             Name of the Dockerfile (Default is 'PATH/Dockerfile')
      --iidfile string          Write the image ID to the file
      --isolation string        Container isolation technology
      --label list              Set metadata for an image
      --network string          Set the networking mode for the RUN instructions during build (default "default")
      --no-cache                Do not use cache when building the image
  -o, --output stringArray      Output destination (format: type=local,dest=path)
      --platform string         Set platform if server is multi-platform capable
      --progress string         Set type of progress output (auto, plain, tty). Use plain to show container output (default "auto")
      --pull                    Always attempt to pull a newer version of the image
  -q, --quiet                   Suppress the build output and print image ID on success
      --secret stringArray      Secret file to expose to the build (only if BuildKit enabled): id=mysecret,src=/local/secret
      --squash                  Squash newly built layers into a single new layer
      --ssh stringArray         SSH agent socket or keys to expose to the build (only if BuildKit enabled) (format: default|<id>[=<socket>|<key>[,<key>]])
  -t, --tag list                Name and optionally a tag in the 'name:tag' format
      --target string           Set the target build stage to build.

- Description for the changelog

* Hide flags that are not supported by BuildKit, if BuildKit is enabled

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

@tonistiigi @tiborvass PTAL