add Dockerfile (#1124)

* add Dockerfile

add Dockerfile

* Update Dockerfile

Co-authored-by: Matthew Peveler <matt.peveler@gmail.com>

Dockerfile

@@ -0,0 +1,21 @@
+FROM ruby:2.5-alpine
+
+RUN apk --no-cache add \
+        g++ \
+        gcc \
+        libc-dev \
+        make \
+        nodejs \
+    && gem install bundler
+
+WORKDIR /srv/slate
+
+COPY . /srv/slate
+
+RUN bundle install
+
+VOLUME /srv/slate/source
+
+EXPOSE 4567
+
+CMD ["bundle", "exec", "middleman", "server", "--watcher-force-polling"]