docker-compose: pin prysm validator to v6.1.1

The prysm_validator service floated on the `stable` tag while prysmctl
and beacon-chain are pinned to v6.1.1. `stable` rolled forward to v7.1.4,
which introduces the `gloas` fork (version 0x07000000). The v6.1.1 genesis
and beacon-chain configs have no gloas entry, so the validator aborts at
startup with "conflicting fork version schedule" and every PoS run fails.

Pin the validator to v6.1.1 to match the rest of the Prysm stack.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: joshuacolvin0

docker-compose.yaml

@@ -139,7 +139,7 @@ services:
   # We run a validator client with 64, deterministically-generated keys that match
   # The validator keys present in the beacon chain genesis state generated a few steps above.
   prysm_validator:
-    image: "gcr.io/prysmaticlabs/prysm/validator:stable"
+    image: "gcr.io/prysmaticlabs/prysm/validator:v6.1.1"
     command:
       - --beacon-rpc-provider=prysm_beacon_chain:5000
       - --datadir=/consensus/validatordata