First prepare the dockerfile for testing

# docker build -t t1 .

FROM ubuntu
# quick install, unattended mode
ENV DEBIAN_FRONTEND=noninteractive
# request port
EXPOSE 22


# Run the install script
# RUN bash /tmp/run.sh
RUN dd if=/dev/zero of=100M.bin count=1 bs=100MB
RUN dd if=/dev/zero of=10M.bin count=1 bs=10MB
RUN rm 100M.bin

CMD ["bash"]

• here use docker build -t t1 . command to build as image t1.

volume calculation.:

1. assume: ubuntu is known to have an initial size of 72.8MB.
2. then the theoretical size of the image should be 72.8+100+10-100=82.8MB.

3. but in reality it is 72.8+100+10=182.8M

   # docker images t1
   REPOSITORY TAG IMAGE ID CREATED SIZE
   t1 latest 7dc3c4768c6e 2 minutes ago 183MB

4. and then look at the file hierarchy

   # docker history t1
   │IMAGE CREATED CREATED BY SIZE COMM
   │ENT
   │7dc3c4768c6e 3 minutes ago /bin/sh -c #(nop) CMD ["bash"] 0B
   │f7977abaef92 3 minutes ago /bin/sh -c rm 100M.bin 0B
   │847d68ab1484 3 minutes ago /bin/sh -c dd if=/dev/zero of=10M.bin count=... 10MB
   │62baa88f2d83 3 minutes ago /bin/sh -c dd if=/dev/zero of=100M.bin count... 100MB
   │2bbf0d43850b 3 minutes ago /bin/sh -c #(nop) EXPOSE 22 0B
   │a6e876477399 3 minutes ago /bin/sh -c #(nop) ENV DEBIAN_FRONTEND=nonin... 0B
   │ba6acccedd29 2 months ago /bin/sh -c #(nop) CMD ["bash"] 0B
   │<missing> 2 months ago /bin/sh -c #(nop) ADD file:5d68d27cc15a80653... 72.8MB
   

   • you can see that rm 100M.bin doesn't really delete the file

the real solution is to initialize it as an instance first, then directly re-export it and then re-import it. This way you can refactor the file hierarchy

# refactor the file hierarchy
function mini_images()
{
    # Original image name
    src=$1
    
    # Temporary container name, do not conflict with existing containers
    dk_tmp=cv.by.kms.app
    
    # Instantiate the container first
    docker create -it -P --name $dk_tmp $src
    
    # Then export the container
    docker export $dk_tmp -o $dk_tmp.tar
    # docker export $dk_tmp |gzip > $dk_tmp.tar.gz
    
    # Delete the original image and the temporary container
    docker rm $dk_tmp
    docker rmi $src
    # Reimport
    docker import $dk_tmp.tar $src
    rm $dk_tmp.tar
    # show the results
    docker images $src
}

# Parameters: Name of the original image (the image you want to refactor)
mini_images t1

• After refactoring and using docker images t1, you can see that the size is reduced to 82.8MB as expected

  # docker images t1
  REPOSITORY TAG IMAGE ID CREATED SIZE
  t1 latest 3f18764312ee 58 seconds ago 82.8MB

you can also use inspect+jq to see the startup information of the original image/container

docker inspect t1|jq
docker inspect t1|jq . [0]|jq .Config.Cmd 
docker inspect t1|jq . [0]|jq .Config.Entrypoint