Declare build-args in build.yml

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>

test/cases/000_build/030_build_args/Dockerfile

@@ -0,0 +1,7 @@
+FROM alpine:3.13
+
+ARG TEST_RESULT=FAILED
+
+RUN echo "printf \"Build-arg test $TEST_RESULT\\n\"" >> check.sh
+
+ENTRYPOINT ["/bin/sh", "/check.sh"]

test/cases/000_build/030_build_args/build.yml

@@ -0,0 +1,7 @@
+image: build-args-test
+network: true
+arches:
+    - amd64
+    - arm64
+buildArgs:
+    - TEST_RESULT=PASSED

test/cases/000_build/030_build_args/test.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+# SUMMARY: Check that the build-args are correctly passed to Dockerfiles
+# LABELS:
+# REPEAT:
+
+set -ex
+
+# Source libraries. Uncomment if needed/defined
+#. "${RT_LIB}"
+. "${RT_PROJECT_ROOT}/_lib/lib.sh"
+
+# Test code goes here
+echo Linuxkig is "$(which linuxkit)"
+RESULT="$(2>&1 linuxkit pkg build --force . | grep PASSED)"
+echo RESULT="${RESULT}"
+echo "${RESULT}" | grep  "Build-arg test PASSED"
+
+exit 0