[RFC] Warn on unknown flags instead of failing

I was trying to optimize a Dockerfile, making use of the experimental --mount=type=cache feature.

# syntax = docker/dockerfile:1-experimental
FROM alpine
RUN --mount=type=cache,target=/var/cache/apk \
  apk add ca-certificates

However, this feature is only available in the experimental syntax, which means that the Dockerfile can no longer be built without buildkit enabled;

DOCKER_BUILDKIT=0 docker build .
Sending build context to Docker daemon  2.048kB
Error response from daemon: Dockerfile parse error line 3: Unknown flag: mount

It would be great to be able to write optimized Dockerfiles (taking advantage of new features), but ignore the new features when using the classic builder.

Looking at the --mount options specifically, ignoring these options (printing a warning) on the classic builder would cause;

I realize this is a tricky one;

• Implementing this may ease the transition to BuildKit; people can start using some of the new features in their Dockerfiles, without necessarily breaking builds for users that have not yet made the transition.
• Although --mount seems to be ok; other new features may not be?
• This could be configurable. Doing so allows a user to opt-out of this behavior (or opt-in), but won't really solve the use-case (writing a Dockerfile with "progressive enhancement")

Note that new commands / Dockerfile instructions would still result in a hard failure.

The payload names no source files or tests. Start by locating the classic-builder Dockerfile parser and its handling of RUN flags, then compare how unknown instructions remain errors. Done means the warning and ignore behavior is defined for unknown flags without changing hard failures for unknown instructions, with tests covering the listed compatibility cases.