Support --perf-test, --shell in barebones test scripts

builder/dockerfile.ubuntu_focal.barebones.deb.test

@@ -2,9 +2,13 @@ FROM ubuntu:focal
 
 ENV STARTUPDIR=/dockerstartup
 
+COPY ./builder/scripts/ /tmp/scripts/
+COPY ./debian/changelog /tmp
+
 ARG KASMVNC_PACKAGE_DIR
 COPY $KASMVNC_PACKAGE_DIR/kasmvncserver_*.deb /tmp/
-RUN rm -f /tmp/kasmvncserver_*+*.deb; apt-get update && dpkg -i /tmp/*.deb; apt-get -yf install
+RUN /tmp/scripts/install_kasmvncserver_package
+
 RUN apt-get update && apt-get -y install xterm lsb-release
 
 RUN mkdir -p $STARTUPDIR