1.8 KiB
Layer by Layer
Objective
Learn about image layers
Requirements
Make sure Docker is installed on your system and the service is started
# Fedora/RHEL/CentOS
rpm -qa | grep docker
systemctl status docker
Instructions
- Write a Dockefile. Any Dockefile! :) (just make sure it's a valid one)
FROM ubuntu
EXPOSE 212
ENV foo=bar
WORKDIR /tmp
RUN dd if=/dev/zero of=some_file bs=1024 count=0 seek=1024
RUN dd if=/dev/zero of=some_file bs=1024 count=0 seek=1024
RUN dd if=/dev/zero of=some_file bs=1024 count=0 seek=1024
- Build an image using the Dockerfile you've wrote
docker image build -t super_cool_app:latest .
- Which of the instructions you've used, created new layers and which added image metadata?
FROM, RUN -> new layer
EXPOSE, ENV, WORKDIR -> metadata
- What ways are there to confirm your answer to the last question?
You can run docker image history super_cool_app. It will show you each instruction and its size. Usually instructions that create new layers has non-zero size, but this is not something you can rely on by itself since, some run commands can have size of zero in docker image history output (e.g. ls -l).
You can also use docker image inspect super_cool_appl and see if in the output, under "RootFS", there are the number of layers that matches the instructions that should create new layers.
- Can you reduce the size of the image you've created?
yes, for example, use all the RUN instructions as a single RUN instruction this way:
RUN dd if=/dev/zero of=some_file bs=1024 count=0 seek=1024 && dd if=/dev/zero of=some_file bs=1024 count=0 seek=1024 && dd if=/dev/zero of=some_file bs=1024 count=0 seek=1024
The change in size might not be dramatic in this case, but in some cases it will make a big impact on the image size.