chore: group single-line directives that are the same

[ci skip]

contrib/build.Dockerfile

@@ -3,9 +3,7 @@ FROM ubuntu:14.04
 ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update
-
 RUN apt-get -y install gcc git build-essential wget ruby-dev ruby1.9.1 lintian rpm help2man man-db
-
 RUN command -v fpm > /dev/null || sudo gem install fpm --no-ri --no-rdoc
 
 ARG GOLANG_VERSION
@@ -18,7 +16,6 @@ ARG WORKDIR=/go/src/github.com/dokku/dokku
 WORKDIR ${WORKDIR}
 
 COPY Makefile ${WORKDIR}/
-
 COPY *.mk ${WORKDIR}/
 
 RUN make deb-setup rpm-setup sshcommand plugn