Renamed some docker files to Dockerfile

This makes it simpler to manage by using the default name. It also helps
my editor to recognise the file and highlight it properly.

docker/3_buildwheels/Dockerfile

@@ -0,0 +1,18 @@
+FROM armadillica/pillar_py:3.6
+LABEL maintainer Sybren A. Stüvel <sybren@blender.studio>
+
+RUN apt-get update && apt-get install -qy \
+    git \
+    build-essential \
+    checkinstall \
+    libffi-dev \
+    libssl-dev \
+    libjpeg-dev \
+    zlib1g-dev
+
+ENV WHEELHOUSE=/data/wheelhouse
+ENV PIP_WHEEL_DIR=/data/wheelhouse
+ENV PIP_FIND_LINKS=/data/wheelhouse
+RUN mkdir -p $WHEELHOUSE
+
+VOLUME /data/wheelhouse