Setting the default `PATH` in the `llb.State` on the client side means it
depends on the `GOOS` of the buildkit client, rather than of the environment
where it will actually execute.
Instead defer this to execution time and insert the default PATH at that point
if one is not present. Doing this in solver/llbsolver/ops/exec covers all
executors and also avoids breaking the cache.
Client compatibility is handled via a new capability.
Fixes#604
Signed-off-by: Ian Campbell <ijc@docker.com>
Akihiro Suda
Ian Campbell
Tonis Tiigi