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Dockerizing Scala apps. Notes.

09 Dec 2016

At some point I realized that building and shipping my software with deb packages sucks a little bit (especially managing a deb repo on a small scale). I have been looking at docker and containers for a some time already and it seemed to be a great idea. When I started working on my new home backup system I decided to give it a try. Since the app I was building was a Scala app, the learning curve was a little bit steeper than for other people. Along the way I noticed following things:

  • docker file can be automatically created in couple ways: by using the sbt-docker plugin, the sbt-native-packager plugin or created manually
  • gitlab hosts its own docker image registry and you can use it for free (yay!)
  • gitlab offers also CI pipelines what makes it super easy to build everything after a push and then automatically deploy to infra

The sbt-native-packager is probably the right way to go in my case as I just want something working and don’t want to configure a lot of things (and this plugin’s philosophy is exactly that). For more complex setups probably sbt-docker is better.

The changes I had to make in build configuration to get the Dockerfile generated correctly:

  • get correct imports: add import com.typesafe.sbt.packager.docker.DockerPlugin.autoImport._
  • enable the plugin: add enablePlugins(DockerPlugin)
  • customize the dockerfile (add the keys from autoImport to settings sequence):
    ...
    dockerExposedPorts := Seq(8080),
    dockerUpdateLatest := true,
    ...

There are some hacks which are not obvious (and helped in my scenario):

  • adding commands in the middle of the dockerfile:

      dockerCommands := {
    val (left, right) = dockerCommands.value.splitAt(3)

    left ++ Seq(
      ExecCmd("RUN", "mkdir", "-p", "/directory/path"),
    ) ++ right
  },

  • pushing to gitlab registry requires some hacks:

      dockerRepository := Some("registry.gitlab.com/..."), // set the repository url
  packageName in Docker := "project-name", // UUUGLY hack - do this if your repo name is different than project name in sbt
  • if you’re using a private repository (as I do) remember to include a docker login ... somewhere before ./sbt docker:publish in the build pipeline

  • when you want to access the image from outside, remember to prefix it with the private repo name!

      $ docker pull registry.gitlab.com/username/image:tag

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