add support for specifying dockerfile in build process

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

test/cases/000_build/055_dockerfiles/003_default/test.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+# SUMMARY: Check that tar output format build is reproducible
+# LABELS:
+
+set -e
+
+# Source libraries. Uncomment if needed/defined
+#. "${RT_LIB}"
+. "${RT_PROJECT_ROOT}/_lib/lib.sh"
+
+linuxkit pkg build --force .
+
+exit 0