Do not fail on missing dockerfile

When a dockerfile is removed, the build CI job fails today as it
see the file in the list of modified ones, but cannot locate it
to run the test.

This still logs the missing file condition, but do not fail, so
that we may pass CI on PRs that delete a dockerfile, like:
- tektoncd#2065
- tektoncd#2064

Signed-off-by: Andrea Frittoli <[email protected]>

tekton/ci/jobs/tekton-image-build.yaml

@@ -47,7 +47,9 @@ spec:
       while [ ! -f "${CONTEXT}/Dockerfile" ]; do
         if [ "${CONTEXT}" == "/" ]; then
           echo "No Dockerfile found in ${FQ_DOCKER_FILE}'s directory or any of its parents"
-          exit 1
+          # The file may be missing if it was deleted as part of the PR
+          # so we stop building but do not fail
+          exit 0
         fi
         CONTEXT=$(dirname ${CONTEXT})
       done