use a single bucket rather than one per conversion

Makefile

@@ -41,6 +41,7 @@ run-server:
 	if [ "$${S3_PUBLIC_ENDPOINT:-}" = "seaweedfs:8333" ]; then S3_PUBLIC_ENDPOINT=localhost:8333; fi; \
 	export S3_ENDPOINT="$${S3_ENDPOINT:-localhost:8333}"; \
 	export S3_PUBLIC_ENDPOINT="$${S3_PUBLIC_ENDPOINT:-localhost:8333}"; \
+	export S3_BUCKET="$${S3_BUCKET:-officeconvert}"; \
 	export S3_USE_SSL="$${S3_USE_SSL:-false}"; \
 	export S3_ACCESS_KEY="$${S3_ACCESS_KEY:-minioadmin}"; \
 	export S3_SECRET_KEY="$${S3_SECRET_KEY:-minioadmin}"; \