RUN --mount=type=cache should inherit ownership/permissions from mountpoint

Start by reproducing the behavior with the Dockerfile example and trace the cache-mount permission handling in BuildKit. The change is done when a cache mount inherits the existing mountpoint's uid/gid when available, while still defaulting to root when the mountpoint does not exist; verify both cases with tests.

When using RUN --mount=type=cache, the uid/gid of the mount defaults to 0:0 (root), even if the mountpoint is an existing directory with different permissions.

While it's possible to manually override the uid/gid for the mount, I think it would make sense to default to the permissions of the mountpoint (if exists), and otherwise fall back to the current behaviour (root).

Example / reproduction steps:

# syntax=docker/dockerfile:1.1.3-experimental

FROM busybox

ARG UID=1000
ARG GID=1000

RUN addgroup -g ${GID} foo
RUN adduser -D -u ${UID} -G foo foo
RUN mkdir -p /mycache && chown -R "${UID}:${GID}" /mycache

# this works (as we're root):
RUN mkdir -p /mycache/root-nomount/bar

# this works (we're still root):
RUN --mount=type=cache,target=/mycache \
        mkdir -p /mycache/root-withmount/bar

USER ${UID}:${GID}

# without `--mount`, this works, because `/mycache` was created with
# the correct permissions:
RUN mkdir -p /mycache/foo-nomount/bar

# with `--mount`, this works if we manually set the uid/gid, but this does not
# work well (currently) with dynamically set UID/GID
RUN --mount=type=cache,target=/mycache,uid=1000,gid=1000 \
        mkdir -p /mycache/foo-withmount-manually/bar

# but this fails:
#
# > [stage-0 9/9] RUN --mount=type=cache,target=/mycache         mkdir -p /mycache/foo-withmount/bar:
#15 0.607 mkdir: can't create directory '/mycache/foo-withmount/': Permission denied
RUN --mount=type=cache,target=/mycache \
        mkdir -p /mycache/foo-withmount/bar

DOCKER_BUILDKIT=1 docker build .

[+] Building 11.9s (15/15) FINISHED                                                                                                                                                           
 => [internal] load build definition from Dockerfile                                                                                                                                     0.1s
 => => transferring dockerfile: 1.07kB                                                                                                                                                   0.0s
 => [internal] load .dockerignore                                                                                                                                                        0.1s
 => => transferring context: 2B                                                                                                                                                          0.0s
 => resolve image config for docker.io/docker/dockerfile:1.1.3-experimental                                                                                                              1.4s
 => docker-image://docker.io/docker/dockerfile:1.1.3-experimental@sha256:888f21826273409b5ef5ff9ceb90c64a8f8ec7760da30d1ffbe6c3e2d323a7bd                                                2.8s
 => => resolve docker.io/docker/dockerfile:1.1.3-experimental@sha256:888f21826273409b5ef5ff9ceb90c64a8f8ec7760da30d1ffbe6c3e2d323a7bd                                                    0.0s
 => => sha256:888f21826273409b5ef5ff9ceb90c64a8f8ec7760da30d1ffbe6c3e2d323a7bd 2.03kB / 2.03kB                                                                                           0.0s
 => => sha256:ee85655c57140bd20a5ebc3bb802e7410ee9ac47ca92b193ed0ab17485024fe5 527B / 527B                                                                                               0.0s
 => => sha256:80b5f664ac0c5f6b89608f7b0afd9548ac5b2d4cd631b7b723d9f2edca8676d9 897B / 897B                                                                                               0.0s
 => => sha256:73e11b97eeae87ce062a224a9ff2d7f8a919d3b50e014a3f33a79b0c219a7003 8.82MB / 8.82MB                                                                                           2.3s
 => => extracting sha256:73e11b97eeae87ce062a224a9ff2d7f8a919d3b50e014a3f33a79b0c219a7003                                                                                                0.3s
 => [internal] load metadata for docker.io/library/busybox:latest                                                                                                                        0.0s
 => [stage-0 1/9] FROM docker.io/library/busybox                                                                                                                                         0.0s
 => => resolve docker.io/library/busybox:latest                                                                                                                                          0.0s
 => [internal] settings cache mount permissions                                                                                                                                          0.1s
 => [stage-0 2/9] RUN addgroup -g 1000 foo                                                                                                                                               0.9s
 => [stage-0 3/9] RUN adduser -D -u 1000 -G foo foo                                                                                                                                      0.9s
 => [stage-0 4/9] RUN mkdir -p /mycache && chown -R "1000:1000" /mycache                                                                                                                 0.8s
 => [stage-0 5/9] RUN mkdir -p /mycache/root-nomount/bar                                                                                                                                 1.0s
 => [stage-0 6/9] RUN --mount=type=cache,target=/mycache         mkdir -p /mycache/root-withmount/bar                                                                                    0.8s
 => [stage-0 7/9] RUN mkdir -p /mycache/foo-nomount/bar                                                                                                                                  0.8s
 => [stage-0 8/9] RUN --mount=type=cache,target=/mycache,uid=1000,gid=1000         mkdir -p /mycache/foo-withmount-manually/bar                                                          0.8s
 => ERROR [stage-0 9/9] RUN --mount=type=cache,target=/mycache         mkdir -p /mycache/foo-withmount/bar                                                                               0.7s
------                                                                                                                                                                                        
 > [stage-0 9/9] RUN --mount=type=cache,target=/mycache         mkdir -p /mycache/foo-withmount/bar:
#14 0.543 mkdir: can't create directory '/mycache/foo-withmount/': Permission denied
------
failed to solve with frontend dockerfile.v0: failed to solve with frontend gateway.v0: rpc error: code = Unknown desc = failed to build LLB: executor failed running [/bin/sh -c mkdir -p /mycache/foo-withmount/bar]: runc did not terminate sucessfully