Fix dockerfile

Previously it tried to do a bundle install without a Gemfile present

Dockerfile

@@ -1,3 +1,6 @@
+# This Dockerfile is optimized for running in development. That means it trades
+# build speed for size. If we were using this for production, we might instead
+# optimize for a smaller size at the cost of a slower build.
 FROM ruby:2.5.3-alpine
 
 # Provide SSL defaults that work in dev/test environments where we do not require connections to secured services
@@ -10,22 +13,18 @@ ENV SETTINGS__SSL__KEY_FILE="${SETTINGS__SSL__KEY_FILE}"
 ENV SETTINGS__SSL__KEY_PASS="${SETTINGS__SSL__KEY_PASS}"
 
 # postgresql-client is required for invoke.sh
-RUN apk --no-cache add \
+RUN apk add --update --no-cache  \
+  build-base \
   postgresql-dev \
   postgresql-client \
   tzdata
 
 RUN mkdir /app
 WORKDIR /app
 
-RUN apk --no-cache add --virtual build-dependencies \
-  build-base \
-  && bundle install --without production \
-  && apk del build-dependencies
-
 COPY Gemfile Gemfile.lock ./
 
-RUN bundle install
+RUN bundle install --without production
 
 COPY . .