docker: split minimum setup to separate script

Dockerfile

@@ -1,10 +1,5 @@
 # https://github.com/cirosantilli/linux-kernel-module-cheat#docker
 FROM ubuntu:18.04
-RUN apt update
-# Minimum requirements to run ./build --download-dependencies
-RUN apt-get install -y \
-      git \
-      python3 \
-      python3-distutils \
-    ;
+COPY setup /
+RUN /setup -y
 CMD bash