arc-testnet: run containers by numeric uid 0 (distroless images)

Circle's arc-execution/arc-consensus images are distroless with only the
`arc` user and no `root` entry in /etc/passwd, so `user: root` failed with
"unable to find user root". Run by numeric uid 0:0 instead (no passwd
lookup needed; root can write the root-owned named volumes).

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

arc/arc/arc-testnet-arc-full.yml

@@ -31,7 +31,7 @@ x-logging-defaults: &logging-defaults
 services:
   arc-testnet-snapshots:
     image: ${ARC_ARC_IMAGE:-docker.cloudsmith.io/circle/arc-network/arc-execution}:${ARC_TESTNET_ARC_VERSION:-0.7.2}
-    user: root
+    user: 0:0
     entrypoint: [/usr/local/bin/arc-snapshots, download]
     command:
       - --chain=arc-testnet
@@ -47,7 +47,7 @@ services:
 
   arc-testnet-consensus-init:
     image: ${ARC_CONSENSUS_IMAGE:-docker.cloudsmith.io/circle/arc-network/arc-consensus}:${ARC_TESTNET_CONSENSUS_VERSION:-0.7.2}
-    user: root
+    user: 0:0
     entrypoint: [/usr/local/bin/arc-node-consensus, init]
     command:
       - --home=/data/consensus
@@ -90,7 +90,7 @@ services:
       net.ipv4.tcp_max_tw_buckets: 2000000        # Allow more TIME_WAIT sockets
     ulimits:
       nofile: 1048576    # Max open files (for RPC/WS connections)
-    user: root
+    user: 0:0
     expose:
       - 8545
       - 9001
@@ -138,7 +138,7 @@ services:
 
   arc-testnet-consensus:
     image: ${ARC_CONSENSUS_IMAGE:-docker.cloudsmith.io/circle/arc-network/arc-consensus}:${ARC_TESTNET_CONSENSUS_VERSION:-0.7.2}
-    user: root
+    user: 0:0
     expose:
       - 31000
       - 29000