Updated changes for Docker extension to sbt-native-packager

- Trait DockerPlugin should not extend DockerKeys
- publishArtifact in Docker is false - do not generate POM/etc.
- Tasks for stage do not depend on each other.
- Change the CWD in the Docker container to install location.
- Adding Docker-specific mappings happens outside mapGenericFilesToDocker
- Use defaultLinuxInstallLocation instead of defaultDockerInstallLocation
- dockerBaseImage is set outside Docker config, easier to override