1. Introduction
This example introduces how to create a Docker container and connect to it. The previous posts discussed creating basic Hello World containers and understanding your Docker installations. This post carries it further to talk about working with containers. This post assumes that you have a working Docker installation. Let’s start
2. Understanding Docker container and image
A Docker container is based on a Docker image. A Docker image is based on a definition provided in a Dockerfile. Consider a Docker image as a read-only snapshot of a Docker Container. The Docker container adds a writable file-system layer on top of a Docker image to make the container usable. Let us understand this a little bit more by examining the official openjdk version 8u111 Docker image. What does its Dockerfile say?
openjdk/8-jdk/Dockerfile (Removed comments from original file for brevity)
FROM buildpack-deps:jessie-scm
RUN apt-get update && apt-get install -y --no-install-recommends \
bzip2 \
unzip \
xz-utils \
&& rm -rf /var/lib/apt/lists/*
RUN echo 'deb http://deb.debian.org/debian jessie-backports main' > /etc/apt/sources.list.d/jessie-backports.list
ENV LANG C.UTF-8
RUN { \
echo '#!/bin/sh'; \
echo 'set -e'; \
echo; \
echo 'dirname "$(dirname "$(readlink -f "$(which javac || which java)")")"'; \
} > /usr/local/bin/docker-java-home \
&& chmod +x /usr/local/bin/docker-java-home
ENV JAVA_HOME /usr/lib/jvm/java-8-openjdk-amd64
ENV JAVA_VERSION 8u111
ENV JAVA_DEBIAN_VERSION 8u111-b14-2~bpo8+1
ENV CA_CERTIFICATES_JAVA_VERSION 20140324
RUN set -x \
&& apt-get update \
&& apt-get install -y \
openjdk-8-jdk="$JAVA_DEBIAN_VERSION" \
ca-certificates-java="$CA_CERTIFICATES_JAVA_VERSION" \
&& rm -rf /var/lib/apt/lists/* \
&& [ "$JAVA_HOME" = "$(docker-java-home)" ]
RUN /var/lib/dpkg/info/ca-certificates-java.postinst configure
Docker builds an image in layers. To understand this, pull the openjdk image locally and see what Docker does.
From the above image, it can be seen that Docker pulled the image in several layers. You can explore what goes into each of these layers by using docker history command
What do you see? The openjdk:8u111 image has 14 layers. These layers are listed in the below table. The details for these layers can be also be seen at Dockerhub
| Dockerfile line | Produces layer | Sized |
|---|---|---|
FROM buildpack-deps:jessie-scm | ADD file:41ea5187c50116884c38d9ec51d920d79cfaeb2a61c52e07a97f457419a10a4f in / | 123 mb |
CMD ["/bin/bash"] | 0 b | |
RUN apt-get update && apt-get install -y --no-install-recommends \ | /bin/sh -c apt-get update && apt-get install -y --no-install-recommends ca-certificates curl wget && rm -rf /var/lib/apt/lists/* | 44.28 mb |
/bin/sh -c apt-get update && apt-get install -y --no-install-recommends bzr git mercurial openssh-client subversion procps && rm -rf /var/lib/apt/lists/* | 122.6 mb | |
/bin/sh -c apt-get update && apt-get install -y --no-install-recommends bzip2 unzip xz-utils && rm -rf /var/lib/apt/lists/* | 1.286 mb | |
RUN echo 'deb http://deb.debian.org/debian jessie-backports main' > /etc/apt/sources.list.d/jessie-backports.list | /bin/sh -c echo 'deb http://deb.debian.org/debian jessie-backports main' > /etc/apt/sources.list.d/jessie-backports.list | 0 b |
ENV LANG C.UTF-8 | ENV LANG C.UTF-8 | 0 b |
RUN { \ | /bin/sh -c { echo '#!/bin/sh'; echo 'set -e'; echo; echo 'dirname "$(dirname "$(readlink -f "$(which javac || which java)")")"'; } > /usr/local/bin/docker-java-home && chmod +x /usr/local/bin/docker-java-home | 87 b |
ENV JAVA_HOME /usr/lib/jvm/java-8-openjdk-amd64 | ENV JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64 | 0 b |
ENV JAVA_VERSION 8u111 | ENV JAVA_VERSION=8u111 | 0 b |
ENV JAVA_DEBIAN_VERSION 8u111-b14-2~bpo8+1 | ENV JAVA_DEBIAN_VERSION=8u111-b14-2~bpo8+1 | 0 b |
ENV CA_CERTIFICATES_JAVA_VERSION 20140324 | ENV CA_CERTIFICATES_JAVA_VERSION=20140324 | 0 b |
RUN set -x \ | /bin/sh -c set -x && apt-get update && apt-get install -y openjdk-8-jdk="$JAVA_DEBIAN_VERSION" ca-certificates-java="$CA_CERTIFICATES_JAVA_VERSION" && rm -rf /var/lib/apt/lists/* && [ "$JAVA_HOME" = "$(docker-java-home)" ] | 351.5 mb |
RUN /var/lib/dpkg/info/ca-certificates-java.postinst configure | /bin/sh -c /var/lib/dpkg/info/ca-certificates-java.postinst configure | 418.2 kb |
So you see how Docker creates layers for every image that you create or pull. Let us see how to create a Docker container and learn further about the containers-and-images story
3. Start a new Docker container from a new image
A new Docker image is created using a Dockerfile. As an example, let us create a new Docker image from the openjdk image we pulled earlier. We will create a Dockerfile that will setup a Helloworld class and execute it. This is the Dockerfile.
FROM openjdk:8u111 COPY . /usr/src/hello-world WORKDIR /usr/src/hello-world RUN javac HelloWorld.java CMD ["java", "HelloWorld"]
Next create a image from this Dockerfile and examine how many new layers are created.
$ docker build -t java-hello-world .
$ docker history java-hello-world:latest
