Bug: stacker convert does not convert Dockerfile FROM-AS correctly

[jmblatten]

stacker version

v1.1.6

Describe the bug

Dockerfile FROM-AS defines a build stage. The FROM is the base for that build stage and the AS gives the build stage a name. The stacker convert does the correct thing with the AS portion, but each "FROM name" becomes "type: docker" and "url: name". The problem is, the FROM can name a build stage in the Dockerfile and not a url to fetch a base image. The stacker build then fails to build such a converted stacker.yaml file.

i.e.
Dockerfile:
FROM docker.io/library/ubuntu:jammy AS my-base
FROM my-base AS A
COPY hello /tmp/hello
FROM A AS B

converted stacker.yaml:

A:
 build_env:
   arch: amd64
 from:
   type: docker
   url: docker://my-base
 imports:
 - dest: /tmp/hello
   path: hello
B:
 build_env:
   arch: amd64
 from:
   type: docker
   url: docker://A
my-base:
 build_env:
   arch: amd64
 from:
   type: docker
   url: docker://docker.io/library/ubuntu:jammy

Attempt a stacker build,
$ stacker build -f stacker.yaml
Creating new OCI Layout at "/home/ubuntu/testrun/oci"
preparing image A...
copying /home/ubuntu/testrun/hello
imported file hashes (after substitutions):
loading docker://my-base
error: couldn't import base layer my-base: initializing source docker://my-base:latest: reading manifest latest in docker.io/library/my-base: requested access to the resource is denied
error: exit status 1

Note that stacker is unable to get "docker://my-base".

Proposed Solution:
Perhaps the "FROM name" portion of a FROM-AS instruction that does not include a ":" could be interpreted as a stacker layer. And thus converted into "type: built" and "tag: name". If ":" is there then convert to "type: docker" and "url: name". When this is done, stacker build works.

To reproduce

See above.

Expected behavior

See above.

Screenshots

No response

Additional context

No response