Scala SBT set dockerRepository "Repository for publishing is not specified"

Question

I'm trying to publish a scala build to a docker registry with the following command :

./sbt "; set dockerRepository := Some(\"registryname/project\"); docker:publish"

I am specifying the dockerRepository but when running the command it is throwing the following error :

[error] You cannot push a "root" repository. Please rename your repository to <user>/<repo> (ex: <user>/example)
java.lang.RuntimeException: Repository for publishing is not specified.
[error] (all/*:publishConfiguration) Repository for publishing is not specified.
[error] (server/docker:publish) Nonzero exit value: 1
[error] (config/*:publishConfiguration) Repository for publishing is not specified.
[error] Total time: 63 s, completed 01/09/2015 10:48:04 AM

When I look at my local docker registry I can see it compiled the docker image but did not prefix the docker registry name :

docker images

REPOSITORY                                     TAG                                           IMAGE ID            CREATED             VIRTUAL SIZE
example                                        0.0.1    SNAPSHOT   a6eb8942aa9f        8 minutes ago       1.136 GB

So it doesn't appear to be accepting the dockerRepository variable I am setting on the command, this is also in the build log :

[15:32:58][Step 4/5] [info] Loading project definition from /build/project
[15:33:01][Step 4/5] [info] Set current project to example (in build file:/build/)
[15:33:06][Step 4/5] [info] Defining {.}/*:dockerRepository
[15:33:06][Step 4/5] [info] The new value will be used by no settings or tasks.
[15:33:06][Step 4/5] [info] Reapplying settings...

Answer 1

From the comment of Alex Laverty:

In build.sbt:

dockerRepository in ThisBuild := Some("docker.io")

or in a project with submodules:

lazy val server = (project in file("server"))
  .settings(dockerRepository := Some("docker.io"),
            dockerUsername := Some("pame"),
            ...)

Then:

sbt docker:publish