Ability to parse args from Dockerfile

We should check if we have args in "FROM" and replace them:

ARG IMAGE=linuxkit/img
FROM ${IMAGE} as src

will be parsed as

FROM linuxkit/img as src

Signed-off-by: Petr Fedchenkov <giggsoff@gmail.com>

test/cases/000_build/041_chained_builds_with_args/build1/Dockerfile

@@ -0,0 +1,5 @@
+ARG IMAGE=alpine
+ARG TAG=3.15
+FROM ${IMAGE}:${TAG}
+
+COPY foo /foo

test/cases/000_build/041_chained_builds_with_args/build1/build.yml

@@ -0,0 +1,5 @@
+image: chained-build-with-args-image1-test-does-not-exist-anywhere-else-123456789 # this must be used as FROM in the chained image
+network: true
+arches:
+    - amd64
+    - arm64

test/cases/000_build/041_chained_builds_with_args/build1/foo

@@ -0,0 +1 @@
+bar

test/cases/000_build/041_chained_builds_with_args/build2/Dockerfile

@@ -0,0 +1,7 @@
+# those IMAGE and TAG *must* be updated when changing ../build1; just run `lkt pkg show-tag ../build1` and place it here
+ARG IMAGE=linuxkit/chained-build-with-args-image1-test-does-not-exist-anywhere-else-123456789
+ARG TAG=6a40ba0866b7ce18b3b13390bf88bfeb6ac0db4e
+FROM ${IMAGE}:${TAG} as src
+FROM alpine:3.15
+COPY --from=src /foo /foo
+RUN cat /foo

test/cases/000_build/041_chained_builds_with_args/build2/build.yml

@@ -0,0 +1,5 @@
+image: chained-build-with-args-test-second
+network: true
+arches:
+    - amd64
+    - arm64

test/cases/000_build/041_chained_builds_with_args/test.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+# SUMMARY: Check that chained builds support args
+# LABELS:
+# REPEAT:
+
+set -ex
+
+# Source libraries. Uncomment if needed/defined
+#. "${RT_LIB}"
+. "${RT_PROJECT_ROOT}/_lib/lib.sh"
+
+# Test code goes here
+echo linuxkit is "$(which linuxkit)"
+
+# build the first, use it to build the second
+linuxkit pkg build --force ./build1
+linuxkit pkg build --force ./build2
+
+exit 0