Table of Contents

• Overview
• Reference
  • Dockerfile
• Terminology
• Topics
  • Dockerfile: ADD vs COPY
• How-to
  • Install docker with a single command
  • Install docker on Amazon Linux instance
  • Access internet from containers when private DNS intrudes
• Links

Overview

Reference

Dockerfile

LABEL

LABEL <key>=<value> <key>=<value> <key>=<value> ...

LABEL "com.example.vendor"="ACME Incorporated"
LABEL com.example.label-with-value="foo"
LABEL version="1.0"
LABEL description="This text illustrates \
that label-values can span multiple lines."

LABEL multi.label1="value1" multi.label2="value2" other="value3"

LABEL multi.label1="value1" \
      multi.label2="value2" \
      other="value3"

MAINTAINER (deprecated)

• Use LABEL

LABEL maintainer="SvenDowideit@home.org.au"

RUN

• RUN <command>
  • shell form, the command is run in a shell, which by default is /bin/sh -c on Linux or cmd /S /C on Windows)
  • You can use a \ (backslash) to continue a single RUN instruction onto the next line.
• RUN ["executable", "param1", "param2"]
  • This, exec form, makes it possible to avoid shell string munging, and to RUN commands using a base image that does not contain the specified shell executable.
• Below is a well-formed RUN instruction that demonstrates all the apt-get recommendations.

RUN apt-get update && apt-get install -y \
    aufs-tools \
    automake \
    build-essential \
    curl \
    dpkg-sig \
    libcap-dev \
    libsqlite3-dev \ mercurial \
    reprepro \
    ruby1.9.1 \
    ruby1.9.1-dev \
    s3cmd=1.1.* \
 && rm -rf /var/lib/apt/lists/*

CMD

• CMD ["executable","param1","param2"] (exec form, this is the preferred form)
• CMD ["param1","param2"] (as default parameters to ENTRYPOINT)
• CMD command param1 param2 (shell form)
• The main purpose of a CMD is to provide defaults for an executing container

COPY

• If you have multiple Dockerfile steps that use different files from your context, COPY them individually, rather than all at once. This will ensure that each step’s build cache is only invalidated (forcing the step to be re-run) if the specifically required files change.

COPY requirements.txt /tmp/
RUN pip install --requirement /tmp/requirements.txt
COPY . /tmp/

Terminology

Topics

Dockerfile: ADD vs COPY

• Although ADD and COPY are functionally similar, generally speaking, COPY is preferred.

• ADD allows <src> to be an URL

• If the <src> parameter of ADD is an archive in a recognised compression format, it will be unpacked

How-to

Install docker with a single command

curl -sSL https://get.docker.com/ | sh

Install docker on Amazon Linux instance

sudo yum update -y
sudo yum install -y docker
sudo service docker start
sudo usermod -a -G docker ec2-user

Access internet from containers when private DNS intrudes

In private network with private DNS, the network administrator often blocks access to common DNS like 8.8.8.8. This may cause to block access to internet within the docker container.

In this case, the private DNS is used within docker containers with --dns option. To use this option by default, put the following line in /etc/default/docker:

# Use DOCKER_OPTS to modify the daemon startup options.
DOCKER_OPTS="--dns 10.10.0.1 --dns 10.10.0.2"

Additionally, systemd script for docker doesn't read /etc/default/docker and $DOCKER_OPTS. To fix it, add following lines to /lib/systemd/system/docker.service:

[Service]
// ...
EnvironmentFile=-/etc/default/docker
ExecStart=/usr/bin/dockerd -H fd:// $DOCKER_OPTS

Links

• docker-cheat-sheet