Minor optimization; put the check for a literal "--" first, instead of
matching for "--" as prefix. Also add some documentation to the code,
to outline why we return early.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

…rrors

Before this change, only the name of the flag would be printed as part
of errors. Change the errors to include the flag-name _including_ the
"--" prefix to make it more clear the error is about a flag that was
provided but invalid.

Before this change:

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

    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

With this change;

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

    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

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>