Build ebpf objects from a standardised container

[rafaelroquetto]

Use a container featuring a standard toolchain to build our ebpf binaries. Quoting cilium/ebpf docs:

We recommend building eBPF C code from within a container with a stable LLVM toolchain, as well as checking all generated .o and .go files into source control. This buys you fully-reproducible builds, prevents bugs due to team members using different LLVM versions and makes your packages fully independent and go runnable. It also prevents PII from leaking into ELFs in the form of absolute paths to .c source files in DWARF info.

[mariomac]

We already have the generator.Dockerfile that already generates that image. It is shipped here: https://github.com/grafana/beyla/pkgs/container/beyla-generator

To build the eBPF artifacts from it, just use: make docker-generate