Don't cache "apt update" in the Dockerfile

Dockerfile.in

@@ -1,6 +1,5 @@
 FROM ubuntu:18.04
-RUN apt update
-RUN apt install -y python3 python3-pip python3-dev python3-sympy build-essential screen sudo psmisc gnuplot-nox
+RUN apt update && apt install -y python3 python3-pip python3-dev python3-sympy build-essential screen sudo psmisc gnuplot-nox
 RUN pip3 install merklelib==1.0
 RUN pip3 install pynacl==1.3.0
 RUN groupadd -g GROUP_ID walkingo