docker: Detect network subdirectory by the presence of 'lightning-rpc'

This is less prune to misdetection compared to using the database file,
but also means that mounting the parent directory will not work if spark
is started before c-lightning is.

Because of this, the documentation was updated to instruct users to
mount the network subdirectory containing the RPC file directly, rather
than relying on the automatic detection mechanism (which is more
advisable anyway).

This appears to be the underlying cause for
btcpayserver/btcpayserver-docker#462,
due to BTCPay's unusual 'rpc-file' setting.

doc/docker.md

@@ -26,11 +26,12 @@ You can use `--no-tls` to turn it off.
 #### With existing `lightningd`
 
 To connect to an existing `lightningd` instance running on the same machine,
-mount the lightning data directory to `/etc/lightning`:
+mount the lightning data subdirectory for the network (e.g. `~/.lightning/bitcoin`)
+into `/etc/lightning`:
 
 ```bash
 $ docker run -it -v ~/.spark-docker:/data -p 9737:9737 \
-             -v ~/.lightning:/etc/lightning \
+             -v ~/.lightning/bitcoin:/etc/lightning \
              shesek/spark-wallet:standalone
 ```
 

scripts/docker-entrypoint.sh

@@ -11,7 +11,7 @@ trap 'jobs -p | xargs -r kill' SIGTERM
 if [ -d /etc/lightning ]; then
   echo -n "Using lightningd directory mounted in /etc/lightning... "
   LN_PATH=/etc/lightning
-  if [ ! -f $LN_PATH/lightningd.sqlite3 ] && [ -f $LN_PATH/$NETWORK/lightningd.sqlite3 ]; then
+  if [ ! -f $LN_PATH/lightning-rpc ] && [ -f $LN_PATH/$NETWORK/lightning-rpc ]; then
     echo -n "Using $LN_PATH/$NETWORK... "
     LN_PATH=$LN_PATH/$NETWORK
   fi