Dockerfile: reduce image size

[ee7]

Exercism has been trying to reduce the size of deployed Docker images. There is a forum thread that lists the size of Docker images for each track.

The size of the exercism/zig-test-runner Dockerfile, as of 2023-08-25T11:00:00Z:

149 MB  /opt/zig/zig
 40 MB  /root/.cache/zig/
 36 MB  /opt/zig/lib/libc/
 12 MB  /opt/zig/lib/include/
 11 MB  /opt/zig/lib/std/
  8 MB  /opt/zig/lib/libcxx/
  4 MB  /lib/libcrypto.so.3
  3 MB  /opt/zig/lib/tsan/
  1 MB  /usr/lib/
  1 MB  /bin/

Total image size: 270 MB

We could shrink the image by doing some combination of:

• Removing every repo file apart from bin/run.sh (4fc9277)
• Removing the /opt/doc/ directory (c9822a1)
• Removing bash (2cbd7d0)
• Removing the biggest libc (ae6ae53)
• Removing more libcs
• Decreasing the size of the Zig executable, by doing one of these:
  • Using a Zig compiler that doesn't depend on LLVM, Clang, and LLD libraries (ziglang/zig#16270)
  • Using a wasm-only Zig compiler, like auguste's zig + zls playground (repo here)
  • Using a x86_64-only Zig compiler. We could build the compiler with e.g. -Denable-llvm=false, then use the x86_64 backend (currently considered experimental), via -fno-llvm -fno-lld.
  • Using a Zig compiler that uses a shared LLVM. See e.g. the Arch Linux PKGBUILD
  • Using a C-only Zig compiler, plus tcc? The C backend is no longer considered experimental. But tcc support isn't expected until Zig 1.1.0.
• Decreasing the size of the Zig cache
• Removing jq from the image (not super worthwhile - we'd have to handle escaping ourselves)
• Using a FROM scratch image, instead of Alpine