bake: fix linking to targets with entitlements

[tonistiigi]

When linked target requires entitlement, same entitlement is also needed by the caller. Otherwise, the request will fail when the build is processed.

Example (before):

target "base" {
  target = "base"
  entitlements = ["network.host"]
}

target "default" {
  contexts = {
    base = "target:base"
  }
}

from alpine as base
run --network=host ip a

from alpine
copy --from=base /etc/passwd /foo

» docker buildx bake
[+] Building 0.0s (1/1) FINISHED                                                                                                     docker-container:gifted_dijkstra
 => [internal] load local bake definitions                                                                                                                       0.0s
 => => reading docker-bake.hcl 138B / 138B                                                                                                                       0.0s
Your build is requesting privileges for following possibly insecure capabilities:

 - Running build containers that can access host network

In order to not see this message in the future pass "--allow=network.host" to grant requested privileges.

Your full command with requested privileges:

docker buildx bake --allow=network.host

Do you want to grant requested privileges and continue? [y/N] y
[+] Building 0.8s (4/5)                                                                                                              docker-container:gifted_dijkstra
 => [default internal] load build definition from Dockerfile                                                                                                     0.0s
 => => transferring dockerfile: 128B                                                                                                                             0.0s
 => [default internal] load metadata for docker.io/library/alpine:latest                                                                                         0.5s
 => [default internal] load .dockerignore                                                                                                                        0.0s
 => => transferring context: 2B                                                                                                                                  0.0s
 => CANCELED [base base 1/2] FROM docker.io/library/alpine:latest@sha256:beefdbd8a1da6d2915566fde36db9db0b524eb737fc57cd1367effd16dc0d06d                        0.1s
 => => resolve docker.io/library/alpine:latest@sha256:beefdbd8a1da6d2915566fde36db9db0b524eb737fc57cd1367effd16dc0d06d                                           0.0s
ERROR: target default: failed to solve: failed to load LLB: network.host is not allowed