Provide RISC-V / `riscv64` Docker images

What should be done?

We should provide RISC-V Docker images.

Go, Golang Docker image, BuildKit, etc – all officially seem to support RISC-V.

Where?

• https://github.com/FerretDB/FerretDB/blob/main/Taskfile.yml

Definition of Done

• FerretDB Docker images for RISC-V are provided.