Thanks @Jan_Koudijs and @Alfred_van_Hoorn. I’d pretty much reached the conclusion that configuring the container through the Docker GUI isn’t possible because the files and folders exposed in the GUI through File Manager are very restricted and don’t allow access to /var (which I presume is a top level folder immediately under root).
However, if I can work out how to get to a command line to run the docker installation, I may still have a go. My thinking is that Synology’s DSM may not support a connection to an external drive, but then neither does DSM support accessing /var, but we know it can be done (Extension Manager running on Synology)
But first I’ll have to work out how to SSH to the server. I guess using PowerShell from my Windows 10 PC is an option … Any pointers welcome