gateway: add hostname option to NewContainer API

[jedevc]

The Meta options struct supports setting the hostname for launching containers, however, it's only used when executing ExecOps. This patch enables setting the hostname for the NewContainer gateway API, instead of just the defaultHostname "buildkitsandbox".

The main use of this is for the debugging feature in buildx, so that the client can set a custom hostname for the --invoked build 🎉 e.g.:

buildx build $PWD --detach=false --invoke sh --builder=dev
INFO: launching local buildx controller
WARNING: No output specified with remote driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load
[+] Building 2.4s (12/12) FINISHED                                                                              remote:dev
 => [internal] load build definition from Dockerfile                                                                  0.0s
 => => transferring dockerfile: 418B                                                                                  0.0s
 => resolve image config for docker.io/docker/dockerfile:1                                                            1.6s
 => [auth] docker/dockerfile:pull token for registry-1.docker.io                                                      0.0s
...
Launching interactive container. Press Ctrl-a-c to switch to monitor console
Interactive container was restarted with process "p1uv38no7xnemyaohn9aous7o". Press Ctrl-a-c to switch to the new container
# bash
root@foo:/# who

where foo is the custom hostname that we could set accordingly in buildx.