Add ability to specify default frontend for a buildkit instance

[bentekkie]

Description

It would be convenient if a buildkit instance could be configured to default to a specific frontend.

The usecase for us is that we have developed a frontend that injects certificates and TLS proxy related environment variables into ExecOps and it would be very helpful if we could configure the buildkit daemon to always use this custom frontend

[bentekkie]

An addition to this is that docker build --build-arg=BUILDKIT_SYNTAX=... ... doesnt seem to work with the dockerfile.v0 frontend disabled

[tonistiigi]

An addition to this is that docker build --build-arg=BUILDKIT_SYNTAX=... ... doesnt seem to work with the dockerfile.v0 frontend disabled

This could be added to buildx as a special case I think. buildctl build --frontend=gateway.v0 --opt source=image should work fine with Dockerfile disabled.

[bentekkie]

An addition to this is that docker build --build-arg=BUILDKIT_SYNTAX=... ... doesnt seem to work with the dockerfile.v0 frontend disabled

This could be added to buildx as a special case I think. buildctl build --frontend=gateway.v0 --opt source=image should work fine with Dockerfile disabled.

Ah ok I will file a FR on the buildx repo for that then

[bentekkie]

Added a FR for #5870 (comment) docker/buildx#3077