Fix PID file permission denied: use squid-writable directory

squid -z (as root) creates /var/run/squid.pid, then gosu drops to
squid user which can't overwrite the root-owned file.

Fixes:
- Test configs: pid_filename /var/run/squid/squid.pid (squid-owned)
- Entrypoint: remove stale PID file before starting as squid user

https://claude.ai/code/session_01Tfy3kPd51qRgxpCFXjb2g9

Author: claude

.github/workflows/squid-build-test.yml

@@ -181,6 +181,7 @@ jobs:
           cache_peer 127.0.0.1 parent 11080 0 no-query no-digest connect-fail-limit=2 connect-timeout=8 round-robin proxy-only originserver name=socks_test socks5
           visible_hostname test
           cache deny all
+          pid_filename /var/run/squid/squid.pid
           cache_log /var/log/squid/cache.log
           access_log stdio:/var/log/squid/access.log combined
           CONF
@@ -335,6 +336,7 @@ jobs:
           cache_peer 127.0.0.1 parent 11081 0 no-query no-digest connect-fail-limit=2 connect-timeout=8 round-robin proxy-only originserver name=socks_auth socks5 socks-user=testuser socks-pass=testpass
           visible_hostname test
           cache deny all
+          pid_filename /var/run/squid/squid.pid
           cache_log /var/log/squid/cache.log
           access_log stdio:/var/log/squid/access.log combined
           CONF

squid_patch/docker-entrypoint.sh

@@ -12,5 +12,8 @@ fi
 # Ensure proper ownership
 chown -R squid:squid /var/cache/squid /var/log/squid /var/run/squid 2>/dev/null || true
 
+# Remove stale PID file left by squid -z (created as root)
+rm -f /var/run/squid.pid /var/run/squid/squid.pid 2>/dev/null || true
+
 echo "Starting Squid with config: ${SQUID_CONFIG_FILE}"
 exec gosu squid squid -N -f "${SQUID_CONFIG_FILE}" "$@"