Add nightly image for testing (#2313)

Author: graham-rp

backend/pkg/testutil/images.go

@@ -113,6 +113,12 @@ func RedpandaImage() string {
 	return getImage("redpanda", "TEST_IMAGE_REDPANDA")
 }
 
+// RedpandaNightlyImage returns the Docker image for Redpanda tests using
+// the nightly image. Can be overridden with TEST_IMAGE_REDPANDA env var.
+func RedpandaNightlyImage() string {
+	return getImage("redpandaNightly", "TEST_IMAGE_REDPANDA")
+}
+
 // KafkaConnectImage returns the Docker image for Kafka Connect tests.
 // Can be overridden with TEST_IMAGE_KAFKA_CONNECT environment variable.
 func KafkaConnectImage() string {

test-images.json

@@ -4,6 +4,10 @@
       "repository": "redpandadata/redpanda",
       "tag": "v25.3.6"
     },
+    "redpandaNightly": {
+      "repository": "redpandadata/redpanda-nightly",
+      "tag": "v0.0.0-20260315git94f368c"
+    },
     "kafkaConnect": {
       "repository": "docker.cloudsmith.io/redpanda/connectors-unsupported/connectors",
       "tag": "latest"