fix(oauth2-proxy): make SIGN_OUT_URL configurable via env var

Apply CodeRabbit nitpick: use ${VAR:-default} pattern for SIGN_OUT_URL
to match REDIRECT_URL convention. Zero-cost consistency improvement.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: SashkoMarchuk

docker-compose.prod.yml

@@ -111,7 +111,7 @@ services:
       - OAUTH2_PROXY_SHOW_DEBUG_ON_ERROR=true
 
       # Logout settings
-      - OAUTH2_PROXY_SIGN_OUT_URL=https://temporal.gluzdov.com/oauth2/sign_out
+      - OAUTH2_PROXY_SIGN_OUT_URL=${OAUTH2_PROXY_SIGN_OUT_URL:-https://temporal.gluzdov.com/oauth2/sign_out}
     healthcheck:
       test: ["CMD", "wget", "-O", "/dev/null", "-q", "http://localhost:8080/oauth2/health"]
       interval: 10s