Change Docker's base image to use alpine. Fixes #5

Dockerfile

@@ -1,6 +1,6 @@
-FROM php:7
+FROM php:7-alpine
 
-RUN apt-get update && apt-get install -y unzip && rm -rf /var/lib/apt/lists/*
+RUN apk info && apk add --no-cache unzip
 
 RUN mkdir /daux && mkdir /build