svc-selkies: honor SELKIES_MODE env var in run script

Selkies's argument parser declares a `mode` setting with `env_var:
SELKIES_MODE` that selects between `websockets` (default) and `webrtc`.
The s6 run script for svc-selkies hardcodes `--mode="websockets"`
which overrides the env var and forces every container into
websocket-mode regardless of the user's `-e SELKIES_MODE=webrtc` setting.

Replace the hardcoded value with `${SELKIES_MODE:-websockets}` so the
existing default is preserved while the documented env var actually
takes effect.

Both the production exec and the DEV_MODE branch are updated for
consistency.

Author: DL6ER

root/etc/s6-overlay/s6-rc.d/svc-selkies/run

@@ -55,12 +55,12 @@ if [ ! -z ${DEV_MODE+x} ]; then
     nodemon -V --ext py --exec \
       "python3" -m selkies \
       --addr="localhost" \
-      --mode="websockets" \
+      --mode="${SELKIES_MODE:-websockets}" \
       --debug="true"
 fi
 
 # Start Selkies
 exec s6-setuidgid abc \
   selkies \
     --addr="localhost" \
-    --mode="websockets"
+    --mode="${SELKIES_MODE:-websockets}"