This allows multiple build flavors for a single codebase, without
sacrificing reproducible builds. The build-args are set in build.yml,
which is typically under the source control (if it is not, then no
reproducible builds are possible anyways). Meaning that mutating
build-args would result in setting "dirty" flag.
Intended use of this commit is to switch between build flavors by
specifying a different yaml file (presumably also under the version
control) by `-build-yml` option.
Because it is impossible to build a final image from packages in
cache, the test for this feature relies on the `RUN echo $build-arg`
output during the `pkg build` process.
Signed-off-by: Yuri Volchkov <yuri@zededa.com>
