Update the CI and deployment to OCaml 5.2.1, Alpine 2.21, opam 2.3, and use BuildKit syntax in the Dockerfile

[MisterDA]

Update the CI and deployment from OCaml 5.2.0 to 5.2.1. Update the deployment image to Alpine 2.21, and opam 2.3 (should be faster).

Updating to the BuildKit syntax should speed up builds a bit, by (among other changes) allowing caching of apk and opam packages. Enabling BuildKit in the deployer also allows parallelism in building the image stages, where possible.

• [x] https://github.com/ocurrent/ocurrent-deployer/pull/248