imagebuildah: default RUN --mount type to bind for cache checksums

[MayukhSobo]

/kind bug

What this PR does / why we need it:

getFromAndSourceKeysFromMountFlag() parses a RUN --mount=... flag to decide whether getCreatedBy() should checksum the mounted content for the build-cache fingerprint. When a Dockerfile omits type= (the normal, idiomatic way to write a bind mount, since bind is the implicit default per the Dockerfile/BuildKit mount syntax), this parser left Type empty instead of defaulting it to "bind" — unlike buildah's other mount parser, the one that actually sets mounts up (internal/volumes/volumes.go's getMounts(), which does default mountType := define.TypeBind).

Because of that, getCreatedBy()'s if mountInfo.Type != "bind" { continue } check skipped checksumming the mounted content for any implicit-bind mount — whether sourced from the build context, another build stage, an image, or an additional build context. Two builds with identical Dockerfile text but different mounted file content ended up with an identical cache key, so the second build silently reused the first build's cached layer instead of picking up the new content.

The fix defaults the parsed mount type to define.TypeBind, matching the default already used by the real mount-setup parser. type=cache and type=tmpfs mounts remain correctly excluded from checksumming — that's intentional, since persisting regardless of build content is their whole purpose.

How to verify it

1. Create a build context with a file, e.g. data.txt containing foo.
2. Build a Dockerfile that uses an implicit bind mount (no type=):

   FROM alpine
   RUN --mount=src=data.txt,target=/tmp/data.txt cat /tmp/data.txt
   

   using buildah build --layers . — it prints foo.
3. Change data.txt to contain bar, then rebuild the same Dockerfile with --layers again.
4. Before this fix: the second build reuses the cached layer and still prints foo, even though the mounted file changed. After this fix: the cache is correctly invalidated and the second build prints bar.

A unit test was also added, TestGetFromAndSourceKeysFromMountFlag in imagebuildah/util_test.go, which fails without this fix (asserts Type defaults to "bind" when type= is omitted) and passes with it:

go test -run TestGetFromAndSourceKeysFromMountFlag ./imagebuildah/...

Which issue(s) this PR fixes:

Fixes podman-container-tools/podman#29126

Special notes for your reviewer:

This bug was reported against podman (originally podman-container-tools/podman#29126, now podman-container-tools/podman#29126 after the org transfer), but root-caused to buildah — podman build vendors buildah's imagebuildah package. There is no separate buildah-side issue tracking this; per CONTRIBUTING.md, a PR alone is sufficient as long as its description includes what an issue report would (repro steps + root cause), which this PR provides above.

Does this PR introduce a user-facing change?

Fixed a build-cache bug where `RUN --mount` flags that omit `type=` (an implicit bind mount) were not checksummed for cache invalidation, causing builds to silently reuse a stale cached layer when only the mounted file's content changed.

[nalind]

LGTM, thanks!

[TomSweeneyRedHat]

Nice! TYVM!
LGTM

[TomSweeneyRedHat]

/lgtm