[Docker] Make sure lightningd receive SIGTERM

In conjunction to https://github.com/ElementsProject/lightning/pull/2172 this fix https://github.com/ElementsProject/lightning/issues/2074 if EXPOSE_TCP is false (which is the case in production)

More info https://github.com/ElementsProject/lightning/pull/2172#issuecomment-447727668

tools/docker-entrypoint.sh

@@ -15,5 +15,5 @@ if [ "$EXPOSE_TCP" == "true" ]; then
     socat "TCP4-listen:$LIGHTNINGD_RPC_PORT,fork,reuseaddr" "UNIX-CONNECT:$LIGHTNINGD_DATA/lightning-rpc" &
     fg %-
 else
-    lightningd "$@"
+    exec lightningd "$@"
 fi