Dockerfile: improve error-messages for invalid flags or flags without required value

[thaJeztah]

Noticed these errors while writing an example;

Example 1: invalid option / flag;

Dockerfile:2
--------------------
   1 |     FROM alpine
   2 | >>> COPY --nosuchflag . .
   3 |
--------------------
ERROR: failed to solve: dockerfile parse error on line 2: unknown flag: nosuchflag

Example 2: flag with missing value;

Dockerfile:4
--------------------
   2 |     FROM alpine
   3 |     WORKDIR /test
   4 | >>> COPY --exclude /.git . .
   5 |
--------------------
ERROR: failed to solve: dockerfile parse error on line 4: missing a value on flag: exclude

I wonder if we could improve that error;

• refer to the flag as --exclude instead of exclude
• perhaps print the expected format (--exclude=<path>)
• suggest a possible fix, e.g. for this example, did you mean --exclude=/.git ?

Last one is always tricky, because COPY allows more than 2 arguments, so COPY /.git . . is technically correct (although /.git overlaps with ., so could be some warning on its own)

[tonistiigi]

@thaJeztah Was this supposed to be closed by #5369?