This is the most exciting feature of Sile to me -- since I currently rely on a slow and memory hungry Java toolchain to convert docbook to pdf with fop -- but I just tried it on two docbook documents (a book and also a short article) and it failed to convert both. Apparently the docbook support is currently very incomplete:

https://github.com/sile-typesetter/sile/issues/1338

Actually it just takes XML as an input format, but depending on your XML you need to provide a class that defines how to typeset each tag. For Docbook SILE has an example class with about 40% of the possible tags defined to something sensible. If you want to typeset Docbook you'd need to round that out to cover your use case. To date it is only supplied as an example of how to process XML.