Creating a Custom Docker Image
WARNING
This tutorial uses examples from our core:java docker image,
which can be found on Github. This tutorial also assumes some knowledge of Docker, we suggest
reading up if this all looks foreign to you.
Creating the Dockerfile
The most important part of this process is to create the Dockerfile
that will be used by the Daemon. Due to heavy restrictions on server containers, you must setup this file in a specific manner.
We try to make use of Alpine Linux as much as possible for our images in order to keep their size down.
# ----------------------------------
# Pterodactyl Core Dockerfile
# Environment: Java
# Minimum Panel Version: 0.6.0
# ----------------------------------
FROM openjdk:8-jdk-alpine
MAINTAINER Pterodactyl Software, <support@pterodactyl.io>
RUN apk add --no-cache --update curl ca-certificates openssl git tar bash sqlite fontconfig \
&& adduser -D -h /home/container container
USER container
ENV USER=container HOME=/home/container
WORKDIR /home/container
COPY ./entrypoint.sh /entrypoint.sh
CMD ["/bin/bash", "/entrypoint.sh"]
Lets walk through the Dockerfile above. The first thing you'll notice is the FROM declaration.
FROM openjdk:8-jdk-alpine
In this case, we are using openjdk:8-jdk-alpine which provides us with Java 8.
Installing Dependencies
The next thing we do is install the dependencies we will need using Alpine's package manager: apk. You'll notice some
specific flags that keep the container small, including --no-cache, as well as everything being contained in a
single RUN block.
Creating a Container User
Within this RUN block, you'll notice the useradd command.
adduser -D -h /home/container container
All Pterodactyl containers must have a user named `container`, and the user home **must** be `/home/container`.
After we create that user, we then define the default container USER
as well as a few ENV settings to be applied to things running
within the container.
Work Directory & Entrypoint
One of the last things we do is define a WORKDIR which
is where everything else will be executed. The WORKDIR must be set the /home/container.
Finally, we need to copy our ENTRYPOINT script into
the docker image root. This is done using COPY, after which
we define the command to be used when the container is started using CMD.
The CMD line should always point to the entrypoint.sh file.
COPY ./entrypoint.sh /entrypoint.sh
CMD ["/bin/bash", "/entrypoint.sh"]
Entrypoint Script
In order to complete this Dockerfile, we will need an entrypoint.sh file which tells Docker how to run this
specific server type.
These entrypoint files are actually fairly abstracted, and the Daemon will pass in the start command as an environment variable before processing it and then executing the command.
#!/bin/bash
cd /home/container
# Output Current Java Version
java -version ## only really needed to show what version is being used. Should be changed for different applications
# Replace Startup Variables
MODIFIED_STARTUP=`eval echo $(echo ${STARTUP} | sed -e 's/{{/${/g' -e 's/}}/}/g')`
echo ":/home/container$ ${MODIFIED_STARTUP}"
# Run the Server
${MODIFIED_STARTUP}
The second command, cd /home/container, simply ensures we are in the correct directory when running the rest of the
commands. We then follow that up with java -version to output this information to end-users, but that is not necessary.
Modifying the Startup Command
The most significant part of this file is the MODIFIED_STARTUP environment variable. What we are doing in this case
is parsing the environment STARTUP that is passed into the container by the Daemon. In most cases, this variable
looks something like the example below:
STARTUP="java -Xms128M -Xmx{{SERVER_MEMORY}}M -jar {{SERVER_JARFILE}}"
You'll notice some placeholders there, specifically {{SERVER_MEMORY}} and {{SERVER_JARFILE}}. These both refer to
other environment variables being passed in, and they look something like the example below.
SERVER_MEMORY=1024
SERVER_JARFILE=server.jar
There are a host of different environment variables, and they change depending on the specific service option configuration. However, that is not necessarily anything to worry about here.
MODIFIED_STARTUP=`eval echo $(echo ${STARTUP} | sed -e 's/{{/${/g' -e 's/}}/}/g')`
The command above simply evaluates the STARTUP environment variable, and then replaces anything surrounded in
curly braces {{EXAMPLE}} with a matching environment variable (such as EXAMPLE). Thus, our STARTUP command:
java -Xms128M -Xmx{{SERVER_MEMORY}}M -jar {{SERVER_JARFILE}}
Becomes:
java -Xms128M -Xmx1024M -jar server.jar
Run the Command
The last step is to run this modified startup command, which is done with the line ${MODIFIED_STARTUP}.