Improve the criteria for adding multistage build

[duaraghav8]

Currently, if the Dockerfile being analysed is single-stage and AI is enabled for Dockershrink, DS will adopt Multistage by creating a final stage.
The original premise behind this was that it is always good to have a "final" stage in Dockerfile which only contains things required at runtime, ie, nodejs runtime + dependencies + app code.

We need to add another condition:

Only adopt multistage if the resultant Dockerfile is doing something meaningful during its build stage.

eg 1:

Original Dockerfile

FROM node:22-alpine

EXPOSE 5000

WORKDIR /app

RUN npm install --omit=dev

CMD ["npm", "start"]

There is no benefit in adding a new "final" stage in this dockerfile.
It is already using a light base image and is essentially only installing dependencies and running the app.

eg 2:

Original Dockerfile

FROM node:22-alpine

EXPOSE 5000

WORKDIR /app

RUN npm install && npm run build

CMD ["npm", "start"]

This dockerfile would genuinely benefit from multistage. The build stage would install all dependencies and run the build processes.
The final stage would perform a fresh dependency install, excluding devDependencies and only run the application.

To sum up, the additional check is:

If the Dockerfile is single-stage and it only installs dependencies and runs the application, then DO NOT adopt multistage.
But if additional tasks are being performed (eg- build, test, lint, format, merge/minify code, etc), then keep these in `build` and put prod dependencies and app run commands into final stage.

Projects analysed:

1. Raneto
2. Haste
3. stiolabs
   (and many more projects! This is a very common pattern)

NOTE: Running DS over these projects without AI did a great job (in line with expectations)

---

Ideal solution for this would be to modify the prompt and tell LLM to follow this.
After some experimentation, it seems like gpt-4o doesn't follow this but o1 does.
Plus DS currently uses gpt 4o 08-06 model version. Upgrading to Nov release REALLY messed up the whole multistage change. So further tests needed.
Consider enhanving the prompt with examples. Input: ..., Output: ...
See branch with changes

Other solutions (more effort involved):
Rule engine or custom ML model to classify the stage is either "needs multistage" or "doesn't need multistage".