dockerfile: add source location to circular dependency error

[tonistiigi]

Update the detection and handling of circular dependency validation.

New output:

# buildctl build --frontend dockerfile.v0 --local context=. --local dockerfile=.
[+] Building 0.0s (1/1) FINISHED
 => [internal] load build definition from Dockerfile                                                                           0.0s
 => => transferring dockerfile: 197B                                                                                           0.0s
Dockerfile:9
--------------------
   7 |
   8 |     FROM alpine AS build
   9 | >>> COPY --from=target /Dockerfile /d2
  10 |
  11 |     FROM target
--------------------
Dockerfile:3
--------------------
   1 |
   2 |     FROM busybox AS target
   3 | >>> COPY --from=build / /out
   4 |
   5 |     FROM target as b
--------------------
error: failed to solve: circular dependency detected on stage: target

[tonistiigi]

@AkihiroSuda Looks like the detachnetns testsuite is causing timeouts?

[AkihiroSuda]

@AkihiroSuda Looks like the detachnetns testsuite is causing timeouts?

Green after restarting.
We have to deflake it though, if its flakiness arises from detach-netns.