image build: use cmake

[pinheadmz]

Closes #672

Seems to be working for me so far locally, will test a few other options as well.

Should we add a test to CI for this? It might just take too long to run...

[willcl-ark]

We do need this, but it will break rebuilding <= v28.0 images...

I think we should simply add a new dockerfile with this in. When combined with #669 having multiple dockerfiles becomes easy to handle IMO. To build v28.0 you'd just run:

docker buildx bake bitcoin-28

and for >28.0 you'd do:

docker buildx bake bitcoin-29
# or perhaps we can get working...?
docker buildx bake <commit sha>

and let the bake magic handle which dockerfile is used for you.

[pinheadmz]

Ah brilliant mate. Will review baker first then rebase this. 👨‍🍳