better caching docker steps

Dockerfile

@@ -1,8 +1,8 @@
-FROM ubuntu:bionic AS frontend
+FROM ubuntu:focal AS frontend
 
 RUN apt-get update && \
     apt-get install -y wget software-properties-common && \
-    echo "deb https://deb.nodesource.com/node_10.x bionic main" | tee /etc/apt/sources.list.d/nodesource.list && \
+    echo "deb https://deb.nodesource.com/node_10.x focal main" | tee /etc/apt/sources.list.d/nodesource.list && \
     wget -q -O- https://deb.nodesource.com/gpgkey/nodesource.gpg.key | apt-key add - && \
     apt-get update && \
     apt-get install -y nodejs
@@ -13,13 +13,12 @@ RUN cd /tmp/code && \
     npm install && \
     ./node_modules/.bin/grunt
 
-FROM ubuntu:disco AS app
-
-ADD . /tmp/code/
+FROM ubuntu:focal AS app
 
 RUN apt-get update && \
     apt-get install -y python3-pip
 
+ADD . /tmp/code/
 COPY --from=frontend /tmp/code/styles/dist/style.css /tmp/code/styles/dist/style.css
 
 RUN cd /tmp/code && \