[chore] speed up examples build using a fixed version of the collector for examples (#38389)

The previous approach created the collector from source on every build,
that is not necessary.

Author: atoulme

exporter/splunkhecexporter/example/Dockerfile

@@ -2,9 +2,7 @@ version: "3"
 services:
 
   otel-collector:
-    build:
-      context: ../../..
-      dockerfile: exporter/splunkhecexporter/example/Dockerfile
+    image: otel/opentelemetry-collector-contrib:0.121.0
     container_name: otel
     command: ["--config=/etc/otel-collector-config.yml"]
     volumes: