add support for building from cached images

Signed-off-by: Avi Deitcher <avi@deitcher.net>

test/cases/000_build/040_chained_builds/build1/Dockerfile

@@ -0,0 +1,3 @@
+FROM alpine:3.15
+
+COPY foo /foo

test/cases/000_build/040_chained_builds/build1/build.yml

@@ -0,0 +1,5 @@
+image: chained-build-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/040_chained_builds/build1/foo

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

test/cases/000_build/040_chained_builds/build2/Dockerfile

@@ -0,0 +1,5 @@
+# this FROM *must* be updated when changing ../build1; just run `lkt pkg show-tag ../build1` and place it here
+FROM linuxkit/chained-build-image1-test-does-not-exist-anywhere-else-123456789:ad06eb29c48f0daab0cc2836a1351a053948100c as src
+FROM alpine:3.15
+COPY --from=src /foo /foo
+RUN cat /foo

test/cases/000_build/040_chained_builds/build2/build.yml

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

test/cases/000_build/040_chained_builds/test.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+# SUMMARY: Check that chained builds get the output of one for the next
+# 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