build: history should not contain ARG values. OTOH RUN history should contain final values.

[flouthoc]

It seems docker never writes ARG values to build history and only writes
values with RUN statements. As a result it can utilize caching better if
assgnment of an ARG value is never used.

Instance

FROM alpine
ARG FOO
ENV FOO=bat
RUN echo $FOO

In above example --build-arg FOO=value has no significance as value
is always overriden by ENV.

Following PR makes sure buildah mimics a similar behaviour.

Closes: #1936

[openshift-ci]

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[rhatdan]

LGTM
@nalind PTAL

[TomSweeneyRedHat]

@flouthoc all kinds of red test unhappiness. I didn't dig, but I'm guessing we've some checks on the args showing values somewhere in the test. Changes look generally good, but definitely want @nalind to take a look.

[flouthoc]

@TomSweeneyRedHat since history does not contains ARG values so we will utilize caching better. Hence older tests were had to be adjusted to expect one less layer. @nalind PTAL.

[TomSweeneyRedHat]

LGTM
@nalind?

[rhatdan]

/lgtm