dockerfile: Support --chmod with non-octal notation, such as --chmod=g=u

[thernstig]

#1492 added support for e.g. --chmod=744 during a COPY command. Here is the original issue for supporting octal notation moby/moby#34819. However, in many situations this is insufficient.

For example, g=u is a way to chmod so that the group gets the same permissions as the user like so: chmod -R g=u dir. This would equal:

COPY --chmod=g=u --from=builder /server/ /server/

This is for example important in OpenShift.

@tobia also added some very valid argumentation for this: moby/moby#34819 (comment):

The usefulness of alpha syntax usually comes from the capital X permission, which means "executable only if the file was previously executable or if it is a directory."

This is because a recursive COPY with --chmod=644 would make all directories non-executable, meaning non-traversable; while a --chmod=755 would make all files executable. Both are inappropriate therefore unuseable 99% of the time.

A recursive COPY with --chmod=u=rwX,go=rX (capital Xes) would set directories to 755 and files to 644, unless a given file already had the executable bit in the source filesystem, in which case it would get 755.

It would be very nice if this support could be added!

[manuschillerdev]

As I currently need this feature I'd love to implement it, if someone can give me a little guidance on it?

[rafrafek]

--umask would be nice too, see comment:
moby/moby#34819 (comment)