Create a diagnostic to validate LABEL's syntax

[rcjsuen]

From what I can tell, LABEL instructions are similar to ENV instructions so we should be able to reuse the parsing code here.

[rcjsuen]

FROM alpine
LABEL a

Error response from daemon: LABEL must have two arguments

FROM alpine
LABEL a=b c

Error response from daemon: Syntax error - can't find = in "c". Must be of the form: name=value

FROM alpine
LABEL a=b c d=e

Error response from daemon: Syntax error - can't find = in "c". Must be of the form: name=value