Add build instructions to WORKSPACE and BUILD

[stepancheg]

I made a mistake (again) of building runsc using bazel directly.

Instruction at the top of BUILD and WORKSPACE file may help.

[konstantin-s-bogom]

It's not necessarily a bad idea to have this, but technically there's nothing stopping you from using Bazel directly (aside from it being broken on the newest version lol). I think after #12027 is merged we can add some extra instructions to the README about which dependencies are needed, and to use bazelisk to manage bazel version for you.

[stepancheg]

there's nothing stopping you from using Bazel directly

My story is: I was trying to build on Ubuntu 25.4 (latest Ubuntu). Binutils for aarch64 no longer includes gold. So build fails with unhelpful message ld not found, while there is ld in $PATH. I figured it out in the end, but it was not easy.

[stepancheg]

we can add some extra instructions to the README about which dependencies are needed

My mistake was not reading readme. I saw bazel files, I presumed it is buildable with bazel. But readme is good enough already: it references Makefile which references dockerfile, which declared dependencies.

Dockerfile is actually how I learned about dependencies: I opened gvisor in GoLand, it suggested opening devcontainers. But then docker on mac somehow could not run runsc, so I created a VM in the cloud; then I just installed all the dependencies listed in the Dockerfile, but it was not enough because of this binutils/gold change in new Ubuntu.

Ideally bazel should bring own toolchains and have no external dependencies, but that seems like significant effort.