Specifics of multi-stage builds in app.yml

[nprimmer]

When performing a multi-stage build within app.yml, the ordering of steps and association with a stage is unclear, at least based on the current documentation:
https://docs.noop.dev/en/articles/7905578-application-manifest-app-yml

• When performing multi-stage builds, is the published container the image in the final stage in the build, or is it the "main" image?
• How can I designate which "stage" a directive takes place in?

The dockerfile equivalent of what I'm trying to do is:
FROM golang:1.21 AS builder
WORKDIR /build
COPY backend/* /build/
RUN go build -o backend ./...

FROM alpine:latest
COPY --from=builder /build/backend /app/
EXPOSE 8080
CMD ["/app/backend"]

[1800joe]

Currently we only support a single base image. The published container is the image in the final step of the build process.

EXPOSE maps to port in the service runtime config and CMD to command also in the service runtime config.

[1800joe]

I spoke too soon. We do support multi-stage builds. See the following example:

The copy step referencing main refers to the original build image.

 - name: WebsiteStatic
    type: service
    image: node:18-alpine
    root: website-static/
    build:
      steps:
        - directory: /website-static
        - copy: package*.json
          destination: ./
        - run: npm ci
        - copy: index.html
        - copy: vite.config.js
        - copy: public/
        - copy: src/
        - run: npm run build
        - image: nginx:1-alpine
          stage: nginx
        - copy: website-static/dist
          destination: /usr/share/nginx/html/
          from: main
        - copy: nginx.conf
          destination: /etc/nginx
    runtime:
      command: 'nginx -g "daemon off;"'
      variables:
        NGINX_ENTRYPOINT_QUIET_LOGS: '1'