what "--from=builder" do in Dockerfile

Question

I am trying to understand the dockerfile for implementing egress-operator. Can someone let me know what does line :

COPY --from=builder /workspace/manager . AND ENTRYPOINT ["/manager"] mean here and what are they trying to do at ENTRYPOINT ["/manager"] ?

Are they running manager script then it should be something like ENTRYPOINT ["./manager"] or they are trying simply cd to /manager folder ?

Answer 1

builder is the name of a previous stage in the multi-stage Dockerfile. It is probably building the application manager. Once the builder is complete, this current stage copies the manager file into the current image /workspace directory. The entrypoint simply gives the program that will run when the container starts.

You can find more detailed explanations here:

https://docs.docker.com/engine/reference/builder/

Answer 2

Normally a COPY command would copy a file from the build context into the image. But with --from it instead copies from the filesystem of a previous build stage. Somewhere above that you'll see FROM whatever AS builder and some build instructions, so it's copying from that.