Add infrastructure for caching downloads in separate image

images/base/Dockerfile

@@ -1,7 +1,10 @@
 FROM ubuntu:23.04
 
+ENV FFBUILD_DLDIR="/opt/ffdl"
+
 ENV DEBIAN_FRONTEND noninteractive
 RUN \
+    mkdir -p "$FFBUILD_DLDIR" && \
     apt-get -y update && \
     apt-get -y dist-upgrade && \
     apt-get -y install build-essential yasm nasm \

images/base/run_stage.sh

@@ -11,5 +11,12 @@ export RAW_LDFLAGS="$LDFLAGS"
 mkdir -p /stage
 source "$1"
 cd /stage
-ffbuild_dockerbuild
+if [[ -n "$3" ]]; then
+    cd "$3"
+fi
+if [[ -z "$2" ]]; then
+    ffbuild_dockerbuild
+else
+    "$2"
+fi
 rm -rf /stage "$FFBUILD_PREFIX"/bin