Document enhanced LABEL/ENV/ARG support

[lphiri]

Provide detailed documentation on how to write rules for checking LABEL/ENV/ARG name/values

[caduvieira]

Just on LABEL layer for image. https://docs.docker.com/engine/reference/builder/#label

[cassiussa]

That only seems to work with this format

LABEL multi.label1="value1"
LABEL multi.label2="value2"
LABEL other="value3"

and not with either of these formats

LABEL multi.label1="value1" \
    multi.label2="value2" \
    other="value3"

LABEL multi.label1="value1" multi.label2="value2" other="value3"

My issue here is that first one (the one that works) produces a new layer for each label.