> For the complete documentation index, see [llms.txt](https://docs.reviactyl.app/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.reviactyl.app/development/eggs/creating-a-custom-docker-image.md). # Creating a Custom Docker Image Creating Docker Image for Reviactyl {% hint style="warning" %} This tutorial uses examples from our [`core:java`](https://github.com/reviactyl/images/tree/master/java) docker image, which can be found on GitHub. This tutorial also assumes some knowledge of [Docker](https://docker.io/), we suggest reading up if this all looks foreign to you. {% endhint %} ## Creating the Dockerfile The most important part of this process is to create the [`Dockerfile`](https://docs.docker.com/engine/reference/builder/) 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](https://alpinelinux.org/) as much as possible for our images in order to keep their size down. ```dockerfile # ---------------------------------- # Reviactyl Core Java Dockerfile # Environment: Java # ---------------------------------- FROM openjdk:8-jdk-alpine MAINTAINER Reviactyl Project, LABEL org.opencontainers.image.source="https://github.com/reviactyl/images" LABEL org.opencontainers.image.licenses=MIT RUN apt-get update -y \ && apt-get install -y lsof curl ca-certificates openssl git tar sqlite3 fontconfig libfreetype6 tzdata iproute2 libstdc++6 \ && useradd -d /home/container -m 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`](https://docs.docker.com/engine/reference/builder/#from) declaration. ```dockerfile FROM openjdk:8-jdk-alpine ``` In this case, we are using [`openjdk:8-jdk-alpine`](https://github.com/docker-library/openjdk) 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`](https://docs.docker.com/engine/reference/builder/#run) block. ## Creating a Container User Within this `RUN` block, you'll notice the `useradd` command. ```bash adduser -D -h /home/container container ``` {% hint style="warning" %} All Reviactyl containers must have a user named `container`, and the user home **must** be `/home/container`. {% endhint %} After we create that user, we then define the default container [`USER`](https://docs.docker.com/engine/reference/builder/#user) as well as a few [`ENV`](https://docs.docker.com/engine/reference/builder/#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`](https://docs.docker.com/engine/reference/builder/#workdir) which is where everything else will be executed. The `WORKDIR` must be set the `/home/container`. Finally, we need to copy our [`ENTRYPOINT`](https://docs.docker.com/engine/reference/builder/#entrypoint) script into the docker image root. This is done using [`COPY`](https://docs.docker.com/engine/reference/builder/#copy), after which we define the command to be used when the container is started using [`CMD`](https://docs.docker.com/engine/reference/builder/#cmd). The `CMD` line should always point to the `entrypoint.sh` file. ```dockerfile 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. ```bash #!/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: ```bash 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. ```bash 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. ```bash 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: ```bash java -Xms128M -Xmx{{SERVER_MEMORY}}M -jar {{SERVER_JARFILE}} ``` Becomes: ```bash 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 `entrypoint.sh` file, on linux you can do this by executing `chmod +x entrypoint.sh` in the directory where the file is. --- # Agent Instructions This documentation is published with GitBook. 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