Fix Docker build to use patched vendor code by setting DEV_OBI=1

This prevents the Dockerfile from running 'make copy-obi-vendor' which
was overwriting our memory spike fixes with the original OBI code.

Author: curusarn

.github/workflows/betterstack-docker.yml

@@ -100,6 +100,7 @@ jobs:
           build-args: |
             VERSION=${{ github.ref_name }}
             COMMIT=${{ github.sha }}
+            DEV_OBI=1
 
       - name: Update Docker Hub description
         if: github.event_name == 'push' && github.ref == 'refs/heads/main'