Docker notes #2

The original Docker logo

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We continue to learn Docker. See the first part on the learning notes.

docker tags

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It works like git tags.

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docker pull python

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Will fetch the python:latest where latest is the default tag. I prefer alpine and python3.6, so if I want to use it:

docker pull python:alpine3-6

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If I want another version of python, I can use corresponding tags in the python repository.

How to access the running web app inside the container?

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Two ways.

• Use the container's internal network IP to access the web server inside the container. (only accessible to docker host)

To learn the IP:

docker inspect --format '{{ .NetworkSettings.IPAddress }}' <container_id>

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inspect command returns lots of useful information about the container.

• Port mapping: This maps the hosts :80 to container's :5000.

docker run -p 80:5000 <container_name>

Volume mapping

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Use case: Keeping databases persisted.

docker run -v /host/data_dir:/container/var/lib/mysql <container_name>

Image History

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docker history <container>

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Let's see how python:alpine3-6 is built:

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(lightsteem-test) ➜  transmitter git:(master) ✗ docker history python:alpine3.6
IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
37daba746bbe        3 months ago        /bin/sh -c #(nop)  CMD ["python3"]              0B
<missing>           3 months ago        /bin/sh -c set -ex;   apk add --no-cache --v…   6.5MB
<missing>           3 months ago        /bin/sh -c #(nop)  ENV PYTHON_PIP_VERSION=10…   0B
<missing>           3 months ago        /bin/sh -c cd /usr/local/bin  && ln -s idle3…   32B
<missing>           3 months ago        /bin/sh -c set -ex  && apk add --no-cache --…   75.8MB
<missing>           3 months ago        /bin/sh -c #(nop)  ENV PYTHON_VERSION=3.6.6     0B
<missing>           4 months ago        /bin/sh -c #(nop)  ENV GPG_KEY=0D96DF4D4110E…   0B
<missing>           4 months ago        /bin/sh -c apk add --no-cache ca-certificates   633kB
<missing>           4 months ago        /bin/sh -c #(nop)  ENV LANG=C.UTF-8             0B
<missing>           4 months ago        /bin/sh -c #(nop)  ENV PATH=/usr/local/bin:/…   0B
<missing>           9 months ago        /bin/sh -c #(nop)  CMD ["/bin/sh"]              0B
<missing>           9 months ago        /bin/sh -c #(nop) ADD file:6edc55fb54ec9fc36…   3.97MB

Pushing your image to docker hub

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• First, log in with docker login.

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docker push <yourusername>/<imagename>

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Looks like the free plan on docker hub has one private repository access.

Writing Dockerfiles

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An example Dockerfile for transmitter project:

FROM python:alpine3.6
RUN apk update
RUN apk add --no-cache gcc python3-dev libc-dev libressl-dev
COPY . /app
RUN mkdir -v '/root/.transmitter/'
RUN mv -v /app/config.json.docker /root/.transmitter/config.json
WORKDIR /app
RUN pip install -e .
ENTRYPOINT ["transmitter"]

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• Use a python3.6 image
• Update the packages
• Install the related packages
• Copy our app to the container
• Create a config dir and copy the example config here.
• Install the app via pip.
• ENTRYPOINT is the command and params will be executed on docker run.

So, if a user builds the transmitter's Dockerfile and run it, transmitter command will run.

Arguments are automatically passed. Ex: docker run -t transmitter enable to enable your witness.

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We have covered the CLI and the important commands. I will be learning docker compose and docker swarm in the next days.