Replace command exec by env var

Docker-DCO-1.1-Signed-off-by: Mangled Deutz <[email protected]> (github: dmp42)

Author: Mangled Deutz

build.sh

@@ -7,7 +7,7 @@ cd $SERVICE_APPROOT
 . ~/env/bin/activate
 
 pip install --download-cache=~/.pip-cache ./depends/docker-registry-core || exit 1
-pip install --download-cache=~/.pip-cache file://`pwd`#egg=docker-registry[bugsnag] || exit 1
+pip install --download-cache=~/.pip-cache file://${PWD}#egg=docker-registry[bugsnag] || exit 1
 
 cp -R * ~/