build: add ulimit support

[crazy-max]

Follow-up moby/buildkit#2398

vendor buildkit: moby/buildkit@23c8aff...ba673bb

FROM busybox
RUN ulimit -n

docker buildx build --ulimit nofile=1062:1062 .
WARN[0000] No output specified for docker-container 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 
#1 [internal] load build definition from Dockerfile
#1 transferring dockerfile: 64B 0.0s done
#1 DONE 0.0s

#2 [internal] load .dockerignore
#2 transferring context: 2B done
#2 DONE 0.0s

#3 [internal] load metadata for docker.io/library/busybox:latest
#3 ...

#4 [auth] library/busybox:pull token for registry-1.docker.io
#4 DONE 0.0s

#3 [internal] load metadata for docker.io/library/busybox:latest
#3 DONE 1.8s

#5 [1/2] FROM docker.io/library/busybox@sha256:f7ca5a32c10d51aeda3b4d01c61c6061f497893d7f6628b92f822f7117182a57
#5 resolve docker.io/library/busybox@sha256:f7ca5a32c10d51aeda3b4d01c61c6061f497893d7f6628b92f822f7117182a57 0.0s done
#5 sha256:24fb2886d6f6c5d16481dd7608b47e78a8e92a13d6e64d87d57cb16d5f766d63 0B / 767.32kB 0.2s
#5 sha256:24fb2886d6f6c5d16481dd7608b47e78a8e92a13d6e64d87d57cb16d5f766d63 767.32kB / 767.32kB 0.2s done
#5 extracting sha256:24fb2886d6f6c5d16481dd7608b47e78a8e92a13d6e64d87d57cb16d5f766d63 0.1s done
#5 DONE 0.3s

#6 [2/2] RUN ulimit -n
#6 0.196 1062
#6 DONE 0.3s