Ahhh - I think I’ve found why this isn’t working for me: I’m running on an older Synology unit (DS1815+) which is limited to DSM 7.1.1 - and this only goes up to Linux kernel 3.10.108.
I’ve tried installing the 2.5 Docker image on a newer Synology (running DSM 7.3, which has Linux kernel 4.4.302) using similar settings to what I was trying - and this seems to start up correctly.
Sooo it looks like I’m falling off the end of software support for this older model. Oh well…