Update Dockerfile to set variable CS2_IP

The variable CS2_IP will be used in entry.sh for cs2 parameter "-ip" to set the tcp listener to a specific ip address.

default: 0.0.0.0 (listen to all IPs on the local machine) is probably the best.
left blank: cs2 try to identify the current ip automatically

bullseye/Dockerfile

@@ -33,6 +33,7 @@ RUN set -x \
 FROM build_stage AS bullseye-base
 
 ENV CS2_SERVERNAME="cs2 private server" \
+    CS2_IP=0.0.0.0 \
     CS2_PORT=27015 \
     CS2_MAXPLAYERS=10 \
     CS2_RCONPW="changeme" \