fix: stacker convert should check if FROMAS base is in current build

[jmblatten]

Currently, stacker converts the base part of a "FROM base AS name"
found in a dockerfile to stacker's from directive with type:docker
and url:docker.
However, the base could be a build stage in the current session.
In that case, stacker's from directive should be type:built and tag:name instead.
This fix cycles thru the build stages to determine which are build stages and which
are bases before converting and sets a flag.

What type of PR is this?
bug

Which issue does this PR fix:
#756

What does this PR do / Why do we need it:
See above.

If an issue # is not available please add repro steps and logs showing the issue:
#756
Steps to reproduce:

myhello.go file contents,

package main

import "fmt"

func main() {
   fmt.Println("hello world!")
}

Dockerfile contents:

FROM golang:1.22 AS mybuild
COPY myhello.go /src/myhello.go
WORKDIR /src
RUN export GOPATH=/go && export PATH=/go/bin:/usr/local/go/bin:$PATH && export HOME=/go && go build -o /bin/myhello ./myhello.go && ls /bin/myhello

FROM alpine AS mybase

FROM mybase AS A
COPY --from=mybuild /bin/myhello /bin/myhello

FROM A AS B
RUN chmod 755 /bin/myhello && \
    /bin/myhello

Resulting, converted stacker.yaml,

A:
  build_env:
    arch: amd64
  from:
    type: docker
    url: docker://mybase
  imports:
  - dest: /bin/myhello
    path: stacker://mybuild/bin/myhello
B:
  build_env:
    arch: amd64
  from:
    type: docker
    url: docker://A
  run:
  - sh -e -c 'chmod 755 /bin/myhello &&     /bin/myhello'
mybase:
  build_env:
    arch: amd64
  from:
    type: docker
    url: docker:alpine
mybuild:
  build_env:
    arch: amd64
  from:
    type: docker
    url: docker:golang:1.22
  imports:
  - dest: /src/myhello.go
    path: myhello.go
  run:
  - mkdir -p /src
  - cd /src
- sh -e -c 'export GOPATH=/go && export PATH=/go/bin:/usr/local/go/bin:$PATH &&
    export HOME=/go && go build -o /bin/myhello myhello.go && ls /bin/myhello'

Note that a stacker build will fail because there isn't a base layer A or B. They are instead stages in a build. Thus they perhaps should be represented as stages in the converted stacker file.

Testing done on this change:

• CI
• Manual:
  Same Dockerfile and myhello.go from above.
  stacker with fix, resulting converted stacker.yaml,

A:
  build_env:
    arch: amd64
  from:
    tag: mybase
    type: built
  imports:
  - dest: /bin/myhello
    path: stacker://mybuild/bin/myhello
B:
  build_env:
    arch: amd64
  from:
    tag: A
    type: built
  run:
  - sh -e -c 'chmod 755 /bin/myhello &&     /bin/myhello'
mybase:
  build_env:
    arch: amd64
  from:
    type: docker
    url: docker:alpine
mybuild:
  build_env:
    arch: amd64
  from:
    type: docker
    url: docker:golang:1.22
  imports:
  - dest: /src/myhello.go
    path: myhello.go
  run:
  - mkdir -p /src
  - cd /src
  - sh -e -c 'export GOPATH=/go && export PATH=/go/bin:/usr/local/go/bin:$PATH &&
    export HOME=/go && go build -o /bin/myhello myhello.go && ls /bin/myhello'

Note that A and B are tagged while mybuild and mybase pull in base images. stacker build should work.

Automation added to e2e:
None

Will this break upgrades or downgrades?
No

Does this PR introduce any user-facing change?:
No, because previously the converted file would not build.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.