Combine apt update and apt install in ref-docker-base

Keeping them in separate RUN layers lets Docker cache the update while
invalidating only the install, so the cached package index can point at
.deb versions that have been superseded on the mirrors.

Author: Nils Bars

ref-docker-base/Dockerfile

@@ -2,9 +2,7 @@ FROM ubuntu:24.04
 
 ARG DEBIAN_FRONTEND=noninteractive
 
-RUN apt update
-
-RUN apt install -y \
+RUN apt update && apt install -y \
     build-essential \
     ca-certificates \
     gcc gcc-multilib g++-multilib \