Fix obsolete verification of source code presence

There is no longer a `setup.py` in the source root, but rather
`pyproject.toml`.  This ensures the Docker container's verification that
the source code seems to be mounted on `/source` is still up to snuff.

Author: lunkwill42

tools/docker/build.sh

@@ -2,7 +2,7 @@
 
 set -ex
 
-if [[ ! -f "/source/setup.py" ]]; then
+if [[ ! -f "/source/pyproject.toml" ]]; then
   echo NAV source code does not appear to be mounted at /source
   exit 1
 fi