README.md

$ sbt -Dredis:6379=propset run
[info] Set current project to test (in build file:/Users/toby/asldifjasklfjsd/)
[info] Running Main
hello world
propset
[success] Total time: 1 s, completed May 19, 2016 1:42:01 PM

build.sbt

name := "test"

scalaVersion := "2.11.7"

libraryDependencies ++= Seq(
  "com.typesafe" % "config" % "1.3.0"
)

src-main-resources-application.conf

redis {
  host = "default"
  host = ${?REDIS_HOST}
  host = ${"redis:6379"}

  port = ${?REDIS_PORT}
  port = 6379

  pool {
    max_connections = 10
    max_connections = ${?REDIS_POOL_MAX_CONNECTIONS}
  }
}

src-main-scala-Main.scala

object Main extends App {
  println("hello world")
  val c = com.typesafe.config.ConfigFactory.load()
  println(c.getString("redis.host"))
}