Remove "exit 0" from prepare script

docker/prepare script is sourced from a another script

the exit 0 instruction was causing the whole script to exit, not
only the docker/prepare script.
Also change a "cd" to a "pushd" and "popd"

Change-Id: I1af230f341cdc0f7209e59894fdd9cfd827d43e9
Merge "Remove "exit 0" from prepare script"