use DOCKER_WORKDIR for git safe.directory

hardcoding the dir /usr/src/app for git safe.directory doesn't
still leaves plugins broken. Change to use $DOCKER_WORKDIR.

flag: none

Test Plan:
- plugin jenkins build generates crystalball prediction
- standard jenkins build generates crystalball prediction

Change-Id: I06b3be6884685377fca3653f18db6a0a0157800d
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/319114
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Andrea Cirulli <andrea.cirulli@instructure.com>
Product-Review: James Butters <jbutters@instructure.com>
Build-Review: James Butters <jbutters@instructure.com>
QA-Review: James Butters <jbutters@instructure.com>

Jenkinsfile

@@ -436,10 +436,11 @@ pipeline {
                         docker exec -dt general-build-container bin/rails graphql:schema
                       '''
 
+                      @SuppressWarnings('GStringExpressionWithinString')
                       def crystalballStep = '''
                         diffFrom=$(git --git-dir $LOCAL_WORKDIR/.git rev-parse $GERRIT_PATCHSET_REVISION^1)
-                        # crystalball will fail without adding /usr/src/app to safe.directory
-                        docker exec -t general-build-container bash -c 'git config --global --add safe.directory /usr/src/app'
+                        # crystalball will fail without adding $DOCKER_WORKDIR to safe.directory
+                        docker exec -t general-build-container bash -c "git config --global --add safe.directory ${DOCKER_WORKDIR%/}"
                         docker exec -dt \
                                         -e CRYSTALBALL_DIFF_FROM=$diffFrom \
                                         -e CRYSTALBALL_DIFF_TO=$GERRIT_PATCHSET_REVISION \