Better Docker handling, added sass compiler to Docker image

Dockerfile

@@ -2,12 +2,18 @@ FROM google/dart:2.7
 
 WORKDIR /app
 
-RUN mkdir /pub-cache
+# Get Dart dependencies
+RUN mkdir -p /pub-cache
 ENV PUB_CACHE=/pub-cache
 ENV PATH="${PATH}:/pub-cache/bin"
 RUN pub global activate webdev
-
 ADD pubspec.* /app/
 RUN pub get
-ADD . /app/
 RUN pub get --offline
+
+RUN apt update && apt install ruby-sass ruby-dev build-essential -y
+RUN gem install sass-listen
+
+ADD . /app/
+
+CMD ["./start.sh"]