MISC: Dockerfile-apt.in: leave the build files around for inspection
MISC: Dockerfile-apt.in: leave the build files around for inspection

The 'make clean' rule is now tested as part of 'distcheck'. For docker
builds it's more convenient to leave all generated files in place for manual
inspection.

Signed-off-by: Tim Janik <6ec6ff229c4e1c5bbd5933ee92e6c5832ff023bb@gnu.org>

tim-janik/rapicorn,tim-janik/rapicorn,tim-janik/rapicorn,tim-janik/rapicorn,tim-janik/rapicorn