Allow default for --output flag.

[sudiptob2]

zstd compression is quite popular compared to the default gzip. But to use zstd, we need to pass several parameters. For example:

docker buildx build --output type=image,oci-mediatypes=true,compression=zstd,push=true,force-compression=true --provenance=false .

We can use Bash aliases or scripts to make this easier, but it would be simpler if buildx allowed setting defaults for --output flag.

A possible implementation has been added to this PR.

[polarathene]

--provenance=false

This is due to the docker specific additions to the index manifest right? It should be avoidable with BuildKit's oci-artifact=true AFAIK.

That said upstream has also discussed intent to switch to both OCI settings as default in future, which would simplify this a bit further for you.