Use 'apt-get' for stable CLI interface

Dockerfile

@@ -2,9 +2,9 @@ FROM ubuntu:20.04
 LABEL maintainer="Max Regan <mgregan2@gmail.com>"
 LABEL description="Container with ARM embedded cross toolchain"
 
-RUN apt update && \
+RUN apt-get update && \
-    apt upgrade -y && \
+    apt-get upgrade -y && \
-    apt install -y \
+    apt-get install -y \
     # Development files
         build-essential \
         git \