docker: use better regex to generate deb-src entries

Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
[AJB: fixed up following dropping emdebian]
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

tests/docker/dockerfiles/debian.docker

@@ -10,7 +10,7 @@
 FROM debian:stable-slim
 
 # Duplicate deb line as deb-src
-RUN cat /etc/apt/sources.list | sed "s/deb/deb-src/" >> /etc/apt/sources.list
+RUN cat /etc/apt/sources.list | sed "s/^deb\ /deb-src /" >> /etc/apt/sources.list
 
 # Install common build utilities
 RUN apt update