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Docker Workshop - Dockerfile

Sections:

What is a Dockerfile

  • Docker can build images automatically by reading the instructions from a Dockerfile.
  • A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image.
  • Using docker build users can create an automated build that executes several command-line instructions in succession.

Usage of docker build

  • The docker build command builds an image from a Dockerfile and a context.
  • The build’s context is the files at a specified location PATH or URL
    • The PATH is a directory on your local filesystem.
    • The URL is a Git repository location.
  • A context is processed recursively
    • So, a PATH includes any subdirectories and the URL includes the repository and its submodules.

Remember in the previous document docker basics we did the following:

$ docker build -t hello-world .
Sending build context to Docker daemon  5.632kB
Step 1/2 : FROM nginx:1.8-alpine
 ---> c0dddb65129b
Step 2/2 : ADD site /usr/share/nginx/html
 ---> Using cache
 ---> 37e37aa0cac0
Successfully built 37e37aa0cac0

Dockerfile Reference Table:

Command Reference Description
ADD Doc Copies files, directories or remote file URLs from src and adds them container
CMD Doc Provide defaults for an executing container
ENTRYPOINT Doc Allows you to configure a container that will run as an executable
ENV Doc Instruction sets the environment variable key to the value value
EXPOSE Doc Instruction informs Docker that the container listens on the specified network ports at runtime
COPY Doc Instruction copies new files or directories from src and adds them to the filesystem of the container at the path dest
LABEL Doc Instruction adds metadata to an image. A LABEL is a key-value pair
VOLUME Doc Creates a shared volume that can be shared among containers or by the host machine
WORKDIR Doc Set the default working directory for the container
FROM Doc The base image to use in the build. This is mandatory and must be the first command in the file.
USER Doc Sets the default user within the container
ARG Doc Instruction defines a variable that users can pass at build-time to the builder with the docker build command using the --build-arg varname=value flag
ONBUILD Doc A command that is triggered when the image in the Dockerfile is used as a base for another image
STOPSIGNAL Doc Instruction sets the system call signal that will be sent to the container to exit
HEALTHCHECK Doc Instruction tells Docker how to test a container to check that it is still working
SHELL Doc Instruction allows the default shell used for the shell form of commands to be overridden
MAINTAINER Doc Instruction sets the Author field of the generated images.

Dockerfile Project

Go Dockerfile:

# Pull base image.
FROM dockerfile/go

LABEL maintainer="marcelbelmont@gmail.com"

# This is deprecated so you should use the LABEL command instead
MAINTAINER Marcel Belmont "marcelbelmont@gmail.com"

ENV GOVERSION 1.8.3

# Add scripts.
ADD bin/go-build /usr/local/bin/go-build
ADD bin/go-run /usr/local/bin/go-run

# Add executable permission to scripts.
RUN chmod +x /usr/local/bin/go-*

# Set instructions on build.
ONBUILD ADD . /gopath/src/app/
ONBUILD RUN go-build

# Define default command.
CMD ["go-run"]

# Expose port.
EXPOSE 8080

Node.js Dockerfile:

#
# Node.js Dockerfile
#
# https://github.com/dockerfile/nodejs
#

# Pull base image.
FROM dockerfile/python

# Install Node.js
RUN \
  cd /tmp && \
  wget http://nodejs.org/dist/node-latest.tar.gz && \
  tar xvzf node-latest.tar.gz && \
  rm -f node-latest.tar.gz && \
  cd node-v* && \
  ./configure && \
  CXX="g++ -Wno-unused-local-typedefs" make && \
  CXX="g++ -Wno-unused-local-typedefs" make install && \
  cd /tmp && \
  rm -rf /tmp/node-v* && \
  npm install -g npm && \
  printf '\n# Node.js\nexport PATH="node_modules/.bin:$PATH"' >> /root/.bashrc

# Define working directory.
WORKDIR /data

# Define default command.
CMD ["bash"]

Understand how CMD and ENTRYPOINT interact

Understand how CMD and ENTRYPOINT interact

  • Both CMD and ENTRYPOINT instructions define what command gets executed when running a container.
  • There are few rules that describe their co-operation.
  • Dockerfile should specify at least one of CMD or ENTRYPOINT commands.
  • ENTRYPOINT should be defined when using the container as an executable.
  • CMD should be used as a way of defining default arguments for an ENTRYPOINT command or for executing an ad-hoc command in a container.
  • CMD will be overridden when running the container with alternative arguments.

EntryPoint Dockerfile:

FROM ubuntu:16.04

COPY debug-mode.sh /root

RUN \
uptime

ENTRYPOINT ["/usr/bin/uptime"]
CMD ["-V"]

ARG Dockerfile:

# ARG is the only command that can precede FROM which is ordinarily first COMMAND in Dockerfile
ARG  CODE_VERSION=latest
FROM base:${CODE_VERSION}
CMD  /code/run-app

FROM extras:${CODE_VERSION}
CMD  /code/run-extras

RethinkDB dockerfile:

#
# RethinkDB Dockerfile
#
# https://github.com/dockerfile/rethinkdb
#

# Pull base image.
FROM dockerfile/ubuntu

# Install RethinkDB.
RUN \
  echo "deb http://download.rethinkdb.com/apt `lsb_release -cs` main" > /etc/apt/sources.list.d/rethinkdb.list && \
  wget -O- http://download.rethinkdb.com/apt/pubkey.gpg | apt-key add - && \
  apt-get update && \
  apt-get install -y rethinkdb python-pip && \
  rm -rf /var/lib/apt/lists/*

# Install python driver for rethinkdb
RUN pip install rethinkdb

# Define mountable directories.
VOLUME ["/data"]

# Define working directory.
WORKDIR /data

# Define default command.
CMD ["rethinkdb", "--bind", "all"]

# Expose ports.
#   - 8080: web UI
#   - 28015: process
#   - 29015: cluster
EXPOSE 8080
EXPOSE 28015
EXPOSE 29015

USER dockerfile:

FROM oracle

USER root

RUN echo 'export ORACLE_SID=XE' >> /etc/bash.bashrc

USER oracle

RUN sqplus @my_schema.sql

STOPSIGNAL dockerfile:

FROM busybox
CMD sleep 3600
STOPSIGNAL SIGKILL

HEALTHCHECK dockerfile:

FROM alpine:3.3

MAINTAINER tomwillfixit

RUN apk update && apk add curl && rm -rf /var/cache/apk/*

COPY helloworld.bin /

EXPOSE 80

HEALTHCHECK --interval=5s --timeout=3s --retries=3 \
      CMD curl -f http://localhost:80 || exit 1

ENTRYPOINT ["/helloworld.bin"]

SHELL dockerfile:

FROM microsoft/windowsservercore

# Executed as cmd /S /C echo default
RUN echo default

# Executed as cmd /S /C powershell -command Write-Host default
RUN powershell -command Write-Host default

# Executed as powershell -command Write-Host hello
SHELL ["powershell", "-command"]
RUN Write-Host hello

# Executed as cmd /S /C echo hello
SHELL ["cmd", "/S"", "/C"]
RUN echo hello

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