(GH-42) Fix Dockerfile

Prior to this commit the docker build process fails with:

`Error: unable to select packages`

This commit fixes the above by using the official ruby image and pinning
the version to 2.7.

Author: chelnak

Dockerfile

@@ -1,7 +1,6 @@
-FROM alpine:3.10
+FROM ruby:2.7-alpine
 
-RUN apk add --no-cache ruby=2.5.7-r0 ruby-json=2.5.7-r0 && \
-    mkdir /puppet-lint /puppet
+RUN mkdir /puppet-lint /puppet
 
 VOLUME /puppet
 WORKDIR /puppet