docker - is a running container created by each RUN command during multistage build?

Question

I'm new to Docker, sorry if my question sounds dumb. below is a dockerfile from a textbook:

FROM diamol/base AS build-stage                  
RUN echo 'Building...' > /build.txt              #line 2

FROM diamol/base AS test-stage
COPY --from=build-stage /build.txt /build.txt
RUN echo 'Testing...' >> /build.txt              #line 6

FROM diamol/base
COPY --from=test-stage /build.txt /build.txt
CMD cat /build.txt

and the author says:

The RUN instruction executes a command inside a container during the build, and any output from that command is saved in the image layer.

My questions are:

Q1-Since the author mentioned RUN instruction executes a command inside a container during the build, does it mean that in the first RUN command in line 2, a contianer is created based on the diamol/base image, and this container modify the content of the files then commit the change back to diamol/base image? and same thing happen in line 6, is my understanding correct?

Q2-If my understanding is correct, isn't that we have three different diamol/base images- the original one and two modified in build-stage and test-stage, isn't it very inefficient because it just a small file(let's say its size is 3kb) inside is differnt, while the image could be as large as 500MB and we have 3 copies which is 500MB*3 = 1.5G?

标签： docker

Answer 1

Q1-The base image is unchanged with FROM diamol/base, you restart from the original image each time. The previous image is used by --from=build-stage. For each step in your build, there is a layer created which can be reused to speed up later build.

Q2-Images are composed of layers, the layers are reused between images. There is maybe a bit of overhead but the space is not duplicated. Images are immutable, the container based on the image contains the changes.

Explanation here

A Docker image is built up from a series of layers. Each layer represents an instruction in the image’s Dockerfile. Each layer except the very last one is read-only.