fix "setpriv: libcap-ng is too old for 'all' caps" error

query available capabilities instead of using "all" caps, as
this fails with kernel 5.9.0

see #46

entrypoint.sh

@@ -12,5 +12,8 @@ echo "WARNING: signal-cli-rest-api runs as signal-api (not as root!)"
 echo "Run 'su signal-api' before using signal-cli!"
 EOF
 
+cap_prefix="-cap_"
+caps="$cap_prefix$(seq -s ",$cap_prefix" 0 $(cat /proc/sys/kernel/cap_last_cap))"
+
 # Start API as signal-api user
-exec setpriv --reuid=1000 --regid=1000 --init-groups --inh-caps=-all signal-cli-rest-api $@
+exec setpriv --reuid=1000 --regid=1000 --init-groups --inh-caps=$caps signal-cli-rest-api $@