AST-1553 Remove IDG from Ubuntu 20 build; Add Ubuntu 24 build

• Since IDG no longer supports Ubuntu 20 (it uses CMake 3.17, while Ubuntu 20 has CMake 3.16), remove it from the Ubuntu 20 CI build.
• Add a basic Ubuntu 24 build on CI.

docker/ubuntu_20_04_base

 FROM ubuntu:20.04
 
 # TODO: needs to be bumped before next DP3 release
-# ENV IDG_VERSION=0.8
 ENV EVERYBEAM_VERSION=v0.5.8
-ENV IDG_VERSION=6b61c038883ad3f807d20047c4f9e1a1f0b8d98a
-ENV AOFLAGGER_VERSION=65d5fba4f4c12797386d3fd9cd76734956a8b233
+ENV AOFLAGGER_VERSION=b1256de90b00a5a83477274390decd6671cdcd38
 
 RUN export DEBIAN_FRONTEND="noninteractive" && \
 	apt-get update && \
@@ -49,15 +47,6 @@ RUN mkdir /aoflagger && cd /aoflagger \
 	&& cmake -G Ninja -DCMAKE_INSTALL_PREFIX=/usr ../src \
 	&& ninja install \
 	&& cd / && rm -rf aoflagger
-# Build IDG
-# (PORTABLE: it may run on a different node than where it was compiled)
-RUN mkdir /idg && cd /idg \
-	&& git clone https://git.astron.nl/RD/idg.git src \
-	&& ( cd src/ && git checkout ${IDG_VERSION} ) \
-	&& mkdir build && cd build \
-	&& cmake -G Ninja -DCMAKE_INSTALL_PREFIX=/usr -DPORTABLE=ON ../src \
-	&& ninja install \
-	&& cd / && rm -rf idg
 # Build EveryBeam
 RUN mkdir /everybeam && cd /everybeam \
 	&& git clone https://git.astron.nl/RD/EveryBeam.git src \