Reduce size of docker image (#217)

Dockerfile

@@ -1,31 +1,27 @@
 FROM ubuntu:18.04
 
-RUN apt-get update -yqq && \
-    apt-get install -yqq --no-install-recommends software-properties-common && \
-    apt-get install -yqq --no-install-recommends sudo curl wget cmake locales git \
-    openssl libssl-dev libjsoncpp-dev uuid-dev zlib1g-dev 
-RUN apt-get install -yqq --no-install-recommends postgresql-server-dev-all
-RUN apt-get install -yqq --no-install-recommends libmariadbclient-dev
-RUN apt-get install -yqq --no-install-recommends libsqlite3-dev
-RUN apt-get install -yqq --no-install-recommends gcc-8 g++-8
+RUN apt-get update -yqq \
+    && apt-get install -yqq --no-install-recommends software-properties-common \
+    sudo curl wget cmake locales git gcc-8 g++-8 \
+    openssl libssl-dev libjsoncpp-dev uuid-dev zlib1g-dev \
+    postgresql-server-dev-all libmariadbclient-dev libsqlite3-dev \
+    && rm -rf /var/lib/apt/lists/* \
+    && locale-gen en_US.UTF-8
 
-RUN locale-gen en_US.UTF-8
-ENV LANG en_US.UTF-8
-ENV LANGUAGE en_US:en
-ENV LC_ALL en_US.UTF-8
+ENV LANG=en_US.UTF-8 \
+    LANGUAGE=en_US:en \
+    LC_ALL=en_US.UTF-8 \
+    CC=gcc-8 \
+    CXX=g++-8 \
+    AR=gcc-ar-8 \
+    RANLIB=gcc-ranlib-8 \
+    IROOT=/install
 
-ENV CC=gcc-8
-ENV CXX=g++-8
-ENV AR=gcc-ar-8
-ENV RANLIB=gcc-ranlib-8
+ENV DROGON_ROOT="$IROOT/drogon"
 
-ENV IROOT=/install
-ENV DROGON_ROOT=$IROOT/drogon
-
-WORKDIR $IROOT
-ADD https://api.github.com/repos/an-tao/drogon/git/refs/heads/master version.json
-RUN git clone https://github.com/an-tao/drogon
+ADD https://api.github.com/repos/an-tao/drogon/git/refs/heads/master $IROOT/version.json
+RUN git clone https://github.com/an-tao/drogon $DROGON_ROOT
 
 WORKDIR $DROGON_ROOT
-RUN ./build.sh
 
+RUN ./build.sh