Besides being more clear about what belongs to which, this helps with
docker caching. The server and ui parts are only rebuilt when their
respective subdirectories change.
Extend this a bit further by making the webpack build not depend on
the target architecture. And adding cache dirs so parts of the server
and ui build process can be reused when layer-wide caching fails.
This replaces the previous Dockerfile, which was a single stage for
building and deployment.
The new one is a multi-stage build. Its "dev" target has the full
development environment; its "deploy" target is more slim. It supports
cross-compiled builds via BuildKit, eg to prepare a build suitable for
a Raspberry Pi:
docker buildx build --load --platform=linux/arm64/v8 --tag=moonfire-nvr --progress=plain --target=deploy -f docker/Dockerfile .
Coming next: updating the installation docs.
