Add a conformance test for copying to a mounted prior stage

[nalind]

What type of PR is this?

/kind other

What this PR does / why we need it:

Add a conformance test for cases where an intermediate stage mounts the contents of a previous stage in a read-write fashion and modifies it, because there isn't a documented expectation for what's supposed to happen.

How to verify it

New conformance test!

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

None

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nalind

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[TomSweeneyRedHat]

Changes LGTM, but a test is failing weirdly. I don't think, it's related, but...

[+1805s] not ok 375 bud with --isolation
[+1805s] # (from function `assert' in file ./helpers.bash, line 430,
[+1805s] #  from function `expect_output' in file ./helpers.bash, line 457,
[+1805s] #  in test file ./bud.bats, line 5641)
[+1805s] #   `expect_output --from="${lines[2]}" "${host_pidns}"' failed
[+1805s] # /var/tmp/go/src/github.com/containers/buildah/tests /var/tmp/go/src/github.com/containers/buildah/tests
[+1805s] # # [checking for: docker.io/library/alpine]
[+1805s] # # [restoring from cache: /var/tmp/buildah-image-cache.10043 / docker.io/library/alpine]
[+1805s] # Getting image source signatures
[+1805s] # Copying blob sha256:9d16cba9fb961d1aafec9542f2bf7cb64acfc55245f9e4eb5abecd4cdc38d749
[+1805s] # Copying config sha256:961769676411f082461f9ef46626dd7a2d1e2b2a38e6a44364bcbecf51e66dd4
[+1805s] # Writing manifest to image destination
[+1805s] # # /var/tmp/go/src/github.com/containers/buildah/tests/./../bin/buildah build --isolation chroot -t testisolation --pid private --signature-policy /var/tmp/go/src/github.com/containers/buildah/tests/./policy.json --file /var/tmp/buildah_tests.gdrfra/my-dir/Containerfile .
[+1805s] # STEP 1/2: FROM alpine
[+1805s] # Trying to pull docker.io/library/alpine:latest...
[+1805s] # Getting image source signatures
[+1805s] # Copying blob sha256:9d16cba9fb961d1aafec9542f2bf7cb64acfc55245f9e4eb5abecd4cdc38d749
[+1805s] # Copying config sha256:961769676411f082461f9ef46626dd7a2d1e2b2a38e6a44364bcbecf51e66dd4
[+1805s] # Writing manifest to image destination
[+1805s] # STEP 2/2: run readlink /proc/self/ns/pid
[+1805s] # pid:[4026531836]
[+1805s] # COMMIT testisolation
[+1805s] # Getting image source signatures
[+1805s] # Copying blob sha256:03901b4a2ea88eeaad62dbe59b072b28b6efa00491962b8741081c5df50c65e0
[+1805s] # Copying blob sha256:9a6a928a276add617402ced2ac9fbbb5726671ba55f9bee46b9f70ef3bd889a8
[+1805s] # Copying config sha256:457f115d16687b5af1ab7b57bb2d8efd839ff1fec44b2f5c550d1b7fad0bca12
[+1805s] # Writing manifest to image destination
[+1805s] # --> 457f115d1668
[+1805s] # Successfully tagged localhost/testisolation:latest
[+1805s] # 457f115d16687b5af1ab7b57bb2d8efd839ff1fec44b2f5c550d1b7fad0bca12
[+1805s] # #/vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
[+1805s] # #|     FAIL: buildah build --isolation chroot -t testisolation --pid private --signature-policy /var/tmp/go/src/github.com/containers/buildah/tests/./policy.json --file /var/tmp/buildah_tests.gdrfra/my-dir/Containerfile .
[+1805s] # #| expected: 'pid:[4026531836]'
[+1805s] # #|   actual: 'Getting image source signatures'
[+1805s] # #\^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[+1805s] # /var/tmp/go/src/github.com/containers/buildah/tests

[nalind]

It could not be related: the new test isn't being run in the CI job that failed.

[rhatdan]

/lgtm