Inconsistencies in RUN mount options parsing/handling

[TyIsI]

While working on a JS/TS Dockerfile generator, I came across some inconsistencies in how the RUN mount options are being parsed/handled.

I just opened an issue for the documentation, but checking the sources, it looks like this might be more source related.

Examples:

## Works with all options that are not listed as supported
RUN --mount=type=tmpfs,target=/cache,size=1024,ro,readonly,rw,readwrite /bin/ls -asl /cache

## Not listed as supported, but pulls from registry
## ERROR [internal] load metadata for docker.io/library/nonexistent:latest
RUN --mount=type=tmpfs,target=/cache,size=1024,from=nonexistent /bin/ls -asl /cache

## Shows directories in root, documentation says from=<stage>, but happily loads image
RUN --mount=type=cache,target=/cache,from=alpine:3,source=/ /bin/ls -asl /cache

I'm not sure if I'm proficient enough in Go to actually come up with a good solution for this and I couldn't find any specs for what the correct behaviour should be, but I believe that the problem lies in parseMount in commands_runmount.go

Happy weekend!

[tonistiigi]

documentation says from=, but happily loads image

from= takes the same argument as FROM <base> or COPY --from=<stage>. It can be named build context, named stage or image name, in that order or preference.