Where is the base docker image specified in Play Framework?

Question

My application is built on Play Framework (2.5.3) using Scala. In order to deploy the app, I create a docker image using sbt docker:publishLocal command. I am trying to figure out where the base docker image file is in the play framework folder structure. I do see a DockerFile in target/docker folder. I don't know how Play Framework creates this DockerFile and where / how Play Framework tells docker to layer the application on the base image. I am scala/play/docker n00b. Please help.

Answer 1

Here are some definitions in order to get an idea of what im talking about.

• Docker containers are stored applications or services that have been configured and stored so that docker can run it on any machine docker is installed on.
• Docker Images are save states of docker container instances which are used to run new containers.
• Dockerfiles are files that contain build instructions for how a docker image should be built. This is for offloading some of the many additional parameters one has to attach in order to run a container just how they want to.

@michaJlS I am looking for details on the base docker image that play framework uses. Regarding your question on why I need it - 1. I like to know where it is 2. I'd like to know how I can add another layer to it if need be.

1.) This is going to be based on what you want to look for. If you are looking for the ready-to-use image for docker, simply do docker images and find what image is implying what you are using. If you are looking for the raw data, you can find that in /var/lib/docker/graph. However this is pointless as you need the image ID which can only be specified by the docker images command i gave above. If the image was not built correctly the image will not appear.

2.) If you wish to modify or attach an additional layer (which by adding layers, you mean to add new files and modules to your application), you need to run the docker image (via the docker run command). Again this is moot if the docker image can not be located by the docker daemon.

While this should answer your question, I do not believe it was what you were asking. People who are concerned with docker are people who want to put applications into containers that can be ran on any platform avoiding dependency issues while maintaining relative convenience. If you are trying to use docker to place your newly created application, you are going to want to build from a dockerfile.

In short, dockerfiles are build instructions that you would normally have to specify either before running a container or when inside one. If you are stating that the sbt docker command created a docker file, your going to need to look for that file and use the docker build command in order to create an instance of your image (info should be provided in that second link). Your best course of action in order to try to get your application to run on a container is to either build it inside a container that has the environment of the running app or simply build it from a dockerfile.

The first option would be to simply be docker run -ti imagename with the image being the environment of your machine that has the running app. You can find some images on the docker hub and one that may be of interest to you is this play-framework image someone else created. That command will put you into an interactive session with the container so you can work as if you are trying to create the app within your own environment. Just don't forget to use docker commit when you are done building!

The faster method of building your image would be to do it from a dockerfile. If you know all the commands and you have all the dependencies needed to create and run your application simply put those into a file named "Dockerfile" (following the instructions and guidance of link 2) and do docker build -t NewImageName /path/to/Dockerfile. This will create an image which you can use to create containers of that image or distribute it how you see fit.