add BUILDER option "script" and OS option "distroless"
adding BUILDER option "script" to support upstream projects
with more than one Dockerfiles,like
it also has ability to support more complex cases.
Options supported by BUILD=script include:
LABEL: the image name
SOURCE_REPO: main project source git repo
SOURCE_REF: git branch or tag for main project source repo
COMMAND: command to run script
SCRIPT: script path
ARGS: args of script (op)
adding OS option "distroless" to support upstream projects
whoes base image is distroless, such as "gcr.io/distroless/base"
Signed-off-by: SidneyAn <firstname.lastname@example.org>