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# Creating a Custom Docker Image


This tutorial uses examples from our core:java (opens new window) docker image, which can be found on GitHub. This tutorial also assumes some knowledge of Docker (opens new window), 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 (opens new window) 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 (opens new window) 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, <[email protected]>

RUN apk add --no-cache --update curl ca-certificates openssl git tar bash sqlite fontconfig \
    && adduser --disabled-password --home /home/container container

USER container
ENV  USER=container HOME=/home/container

WORKDIR /home/container

COPY ./ /

CMD ["/bin/bash", "/"]

Lets walk through the Dockerfile above. The first thing you'll notice is the FROM (opens new window) declaration.

FROM openjdk:8-jdk-alpine

In this case, we are using openjdk:8-jdk-alpine (opens new window) 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 (opens new window) 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 (opens new window) as well as a few ENV (opens new window) settings to be applied to things running within the container.

# Work Directory & Entrypoint

One of the last things we do is define a WORKDIR (opens new window) which is where everything else will be executed. The WORKDIR must be set the /home/container.

Finally, we need to copy our ENTRYPOINT (opens new window) script into the docker image root. This is done using COPY (opens new window), after which we define the command to be used when the container is started using CMD (opens new window). The CMD line should always point to the file.

COPY ./ /
CMD ["/bin/bash", "/"]

# Entrypoint Script

In order to complete this Dockerfile, we will need an 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.

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

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:


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.


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}}


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}.

# Note

Sometimes you may need to change the permissions of the file, on linux you can do this by executing chmod +x in the directory where the file is.