Apply Docker conventions for Dockerfile

Dockerfile

@@ -1,11 +1,21 @@
 FROM debian:8
 
 RUN apt-get update
-RUN apt-get install -y libwebp5 libfontconfig libjpeg62 libssl1.0.0 libicu52 curl
+
+RUN apt-get install -y \
+    curl \
+    libwebp5 \
+    libfontconfig \
+    libjpeg62 \
+    libssl1.0.0 \
+    libicu52
 
 COPY . /decktape
 
 WORKDIR /decktape
+
+RUN mkdir bin
+
 RUN curl \
     -L https://astefanutti.github.io/decktape/downloads/phantomjs-linux-debian8-x86-64 \
     -o bin/phantomjs