Keep package index files small

This is mostly for those cases where Dockerfile RUN instructions do
not run `remove-apt-lists`, either by accident or on purpose.

scripts/tweak-apt-config

@@ -21,6 +21,12 @@ APT::Update::Post-Invoke { "if test -x $cmd; then $cmd; fi"; };
 EOF
 chmod 0644 $cfg/docker-clean
 
+cat > $cfg/docker-compress <<- EOF
+Acquire::GzipIndexes "true";
+Acquire::CompressionTypes::Order { "gz"; };
+EOF
+chmod 0644 $cfg/docker-compress
+
 cat > $cfg/docker-translations <<- EOF
 Acquire::Languages "none";
 EOF