volumes: Implement subpath mount #7187

Job	Run time
validate-dco / run	21s
validate-dco / run	21s
build	2m 5s
build	2m 5s
test (dockerd, client, integration)	14m 24s
test (dockerd, client, integration)	14m 24s
test (dockerd, cmd/buildctl, integration)	5m 50s
test (dockerd, cmd/buildctl, integration)	5m 50s
test (dockerd, solver, integration)	5m 55s
test (dockerd, solver, integration)	5m 55s
test (dockerd, frontend, integration)	6m 1s
test (dockerd, frontend, integration)	6m 1s
test (dockerd, frontend/dockerfile, integration)	17m 48s
test (dockerd, frontend/dockerfile, integration)	18m 29s
test (dockerd-containerd, client, integration)	15m 50s
test (dockerd-containerd, client, integration)	15m 50s
test (dockerd-containerd, cmd/buildctl, integration)	5m 52s
test (dockerd-containerd, cmd/buildctl, integration)	5m 52s
test (dockerd-containerd, solver, integration)	5m 38s
test (dockerd-containerd, solver, integration)	5m 38s
test (dockerd-containerd, frontend, integration)	5m 54s
test (dockerd-containerd, frontend, integration)	5m 54s
test (dockerd-containerd, frontend/dockerfile, integration)	19m 55s
test (dockerd-containerd, frontend/dockerfile, integration)	19m 55s
	3h 31m 47s

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