Not all dockerfiles are valid just because all instructions in the
dockerfile are individually valid. For example, a RUN instruction may
not precede the first FROM instruction and so on. The new rule DL3061
checks that the order of the instructions is not completely bogus. That
means the Dockefile must begin with a comment, an ARG or a FROM
instruction.

• Add new rule DL3061 to check for instruction order
• Add tests for rule DL3061
• Fix regression tests wrt. rule DL3061

fixes: #737

How to verify it

The new rule should alert in case the beginning of a dockerfile messes up instruction order such that a build can not succeed, e.g.:

RUN foo bar  # this is bad
FROM alpine:3
RUN bar foo

But should of course not alert in case a Dockerfile is valid and can be build:

# comments are ok
ARG BUILD_ARGUMENTS_OK=true
FROM alpine:3  # FROM must precede other instructions
LABEL foo "bar" # this is ok now
RUN something  # other instructions can follow

Thanks to @ye for suggesting this rule.