55 lines
1.8 KiB
Markdown
55 lines
1.8 KiB
Markdown
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## Layer by Layer
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### Objective
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Learn about image layers
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### Requirements
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Make sure Docker is installed on your system and the service is started
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```
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# Fedora/RHEL/CentOS
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rpm -qa | grep docker
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systemctl status docker
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```
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### Instructions
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1. Write a Dockefile. Any Dockefile! :) (just make sure it's a valid one)
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```
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FROM ubuntu
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EXPOSE 212
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ENV foo=bar
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WORKDIR /tmp
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RUN dd if=/dev/zero of=some_file bs=1024 count=0 seek=1024
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RUN dd if=/dev/zero of=some_file bs=1024 count=0 seek=1024
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RUN dd if=/dev/zero of=some_file bs=1024 count=0 seek=1024
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```
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2. Build an image using the Dockerfile you've wrote
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`docker image build -t super_cool_app:latest .`
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3. Which of the instructions you've used, created new layers and which added image metadata?
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```
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FROM, RUN -> new layer
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EXPOSE, ENV, WORKDIR -> metadata
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```
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4. What ways are there to confirm your answer to the last question?
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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`).
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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.
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5. Can you reduce the size of the image you've created?
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yes, for example, use all the RUN instructions as a single RUN instruction this way:
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`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`
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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.
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