Custom Docker Image

Image is a file package including applications, system function libraries for applications and configurations. Creating a custom image is the process of making a package like this.

Layer

To make a java application image, we may need to include

Ubuntu base image
JRE env variables
Jar package
A script for launching (e.g. java -jar xx.jar) And finally get a Java application.

Each of the above items forms a layer. All layers forms the image.

An advantage of seperating an image into layers is that the layers can be shared among different images.

Two common layers are:

BaseImage (system functions, env, conf, etc. The layer at the bottom)
EntryPoint (usually the script and parameters to start the applications. The layer at the top)

A good news: we don't need to construct all layers by ourselves. We can use dockerfile to tell docker the structure of our image. Docker will make the image for us.

Dockerfile

Common Docker Instructions

A dockerfile contains many docker instructions. Here are some common ones.

Instruction	Desc	e.g.
`FROM`	choose the base image	`FROM centos:6`
`ENV`	set env variables, such as time zone	`ENV key value`
`COPY`	copy local file to a directory of the image	`COPY ./jrell.tar.gz /tmp`
`RUN`	execute linux shell commands	`RUN ls`
`EXPOSE`	specify the port that will be listened. This is a clarification for the user of the container, who still need to use `-p` to configure ports.	`EXPOSE 8080`
`ENTRYPOINT`	the command to be called when the container starts	`ENTRYPOINT java -jar xx.jar`

More syntax

An Example

First, this version does everything from scratch:

FROM ubuntu:16.04
ENV JAVA_DIR=/usr/local
COPY ./jdk8.tar.gz $JAVA_DIR/
COPY ./docker-demo.jar /tmp/app.jar
RUN cd $JAVA_DIR \ && tar -xf ./jdk8.tar.gz \ && mv ./jdk1.8.0_144 ./java8
ENV JAVA_HOME=$JAVA_DIR/java8
ENV PATH=$PATH:$JAVA_HOME/bin
ENTRYPOINT ["java", "-jar", "/app.jar"]

This version is simpler, based on JDK image:

FROM openjdk:11.0-jre-buster
COPY docker-demo.jar /app.jar
ENTRYPOINT ["java", "-jar", "/app.jar"]

Building the Image

Once we have a dockerfile, we can build an image using a command.

docker build -t myImage:1.0 .

-t gives the image a name, :1.0 means version (default: latest)
. specifies the directory that dockerfile is in

Then you can check the built image using docker images.