Hide --dockerfile flag

[mattt]

Follow-up to #1229

The --dockerfile flag is intended primarily for debugging, and shouldn't be advertised in command usage output. This PR marks the flag as hidden.

$ cog build --help
Build an image from cog.yaml

Usage:
  cog build [flags]

Flags:
  -h, --help                         help for build
      --no-cache                     Do not use cache when building the image
      --openapi-schema string        Load OpenAPI schema from a file
      --progress string              Set type of build progress output, 'auto' (default), 'tty' or 'plain' (default "auto")
      --secret stringArray           Secrets to pass to the build environment in the form 'id=foo,src=/path/to/file'
      --separate-weights             Separate model weights from code in image layers
  -t, --tag string                   A name for the built image in the form 'repository:tag'
      --use-cuda-base-image string   Use Nvidia CUDA base image, 'true' (default) or 'false' (use python base image). False results in a smaller image but may cause problems for non-torch projects (default "auto")

Global Flags:
      --debug     Show debugging output
      --version   Show version of Cog

[technillogue]

I would be in favor of letting people in general use it, maybe with a disclaimer saying the entrypoint needs to work and conform

[mattt]

There's symmetry between cog build --dockerfile= and cog debug, which is similarly hidden. Whatever we do for one we should do for both.

[technillogue]

It's also a shame cog debug is hidden