Upgrade Docker image

When this Dockerfile was originally defined there was not a pre-built
compose image. Having that available dramatically simplifies the
Dockerfile here. The need for upgrade comes from the fact that the
previous version of Alpine was bringing along a very old version or Ruby
(2.3) which is not longer supported and some gems were failing to
install.

Dockerfile

@@ -1,75 +1,25 @@
-FROM alpine:3.5
-
-ARG DOCKER_VERSION=1.13.1
-ARG COMPOSE_VERSION=1.11.2
+FROM docker/compose:1.19.0
 
 WORKDIR /usr/src/app
 
 COPY . .
 
 RUN \
-  # install dev dependencies
-  apk --no-cache add \
+  apk --no-cache add --virtual .rundeps \
     bash \
     build-base \
+    curl \
+    docker \
     git \
     libffi-dev \
-    py-pip \
-    python \
     ruby \
     ruby-bundler \
     ruby-dev \
-    openssh \
-    wget && \
-
-  # install docker
-  mkdir /code && \
-  cd /code && \
-  wget \
-    --no-check-certificate \
-    -qO- \
-    https://get.docker.com/builds/Linux/x86_64/docker-${DOCKER_VERSION}.tgz \
-    | tar xvz && \
-  mv docker/docker /usr/local/bin && \
-
-  # install docker-compose
-  git \
-    clone \
-    --branch ${COMPOSE_VERSION} \
-    https://github.com/docker/compose.git \
-    /code/compose && \
-  cd /code/compose && \
-  pip \
-    --no-cache-dir install \
-    -r requirements.txt \
-    -r requirements-dev.txt \
-    pyinstaller==3.1.1 && \
-  git rev-parse --short HEAD > compose/GITSHA && \
-  ln -s /lib /lib64 && \
-  ln -s /lib/libc.musl-x86_64.so.1 ldd && \
-  ln -s /lib/ld-musl-x86_64.so.1 /lib64/ld-linux-x86-64.so.2 && \
-  pyinstaller docker-compose.spec && \
-  unlink /lib/ld-linux-x86-64.so.2 /lib64 ldd || true && \
-  mv dist/docker-compose /usr/local/bin/docker-compose && \
-  chmod +x /usr/local/bin/docker-compose && \
-
-  # install ruby dependencies
-  bundle install --gemfile=/usr/src/app/Gemfile --clean -j4 && \
-
-  # cleanup
-  pip freeze | xargs pip uninstall -y && \
+    ruby-json
 
-  apk del \
-    bash \
-    build-base \
-    ca-certificates \
-    libffi-dev \
-    py-pip \
-    python \
-    ruby-dev && \
+RUN \
+  bundle install --gemfile=/usr/src/app/Gemfile --clean -j4
 
-  rm -rf /code /usr/lib/python2.7/ /root/.cache /var/cache/apk/* && \
-  rm -rf /usr/lib/ruby/gems/*/cache/* && \
-  rm -rf /var/cache/apk/* /tmp
+ENTRYPOINT []
 
 CMD rake rspec:unit