feat: add Dockerfile support

[setchy]

#657

[setchy]

Opening a DRAFT PR early for feedback.

It's pretty rough, but appears to be functional locally when using a sample Dockerfile, eg:

FROM eclipse-temurin:latest AS JAVA

FROM node:20 AS build

FROM node:18.0.0@sha256:7b58c794e37aa3e6f6df5f9616eaa0bea27dedb12510e962ba6f505d17d8ffd3 AS SOURCE

[setchy]

Testing locally, it seems like only one SrcFile is added to an image when the image may exist across multiple Dockerfiles/Containerfiles.

Need to do some further debugging as to why this is happening...

[setchy]

A few updates

• updated logic to to skip processing any image statements which reuse a previous build stage name.
• created demo/test repository: https://github.com/setchy/cdxgen-dockerfile-demo
• Split out the multiple SrcFile support into a separate PR: feat: oci image support for multiple src files #696

While debugging, i noticed that dependencies were being found and logged, however they are not surviving through to the final output bom, which as I understand affects things like Dependency Graph within Dependency Track. This needs some further investigation...

[setchy]

I've split the dependencies enhancement into #697 to help streamline this PR

[setchy]

re-raised as #704