Fix linting reported by super-linter (#37)

Dockerfile

@@ -1,18 +1,24 @@
 FROM ruby:2.6.4-alpine3.9
 
-LABEL maintainer "[email protected]"
-RUN apk add --no-cache git
+LABEL maintainer="[email protected]"
 
+SHELL ["/bin/ash", "-eo", "pipefail", "-c"]
+
+RUN apk add --no-cache \
+  git=2.20.4-r0
+
+WORKDIR /
 COPY Gemfile Gemfile
-RUN gem install bundler --version $(cat Gemfile | grep bundler | awk -F "'" '{print $4}') \
+RUN gem install bundler --version "$(grep bundler Gemfile | awk -F "'" '{print $4}')" \
   && bundle install --system \
-  && gem uninstall bundler
+  && gem uninstall bundler \
+  && rm Gemfile
 
 ENV SRC_PATH /usr/local/src/your-app
-RUN mkdir -p $SRC_PATH
+RUN mkdir -p "${SRC_PATH}"
 
 VOLUME [ "$SRC_PATH" ]
 WORKDIR $SRC_PATH
 
-CMD ["--help"]
 ENTRYPOINT ["github_changelog_generator"]
+CMD ["--help"]