exec: allow content-cache for root selected mounts

These mounts are actually safe, as suggested by Erik on slack:

> Is it correct that this wouldn’t be a problem in the case where the
> selector of the mount is just “/“? Because then there’s no “hidden”
> files.
>
> If so, maybe there’s a path to enabling content cache for rw mounts
> that are from “/“. Then you could also get the same end behavior by
> not using selectors and instead always copying the subdir you want to
> mount to scratch.

This patch adds a check for this case, and explicitly enables
content-based cache for these cases.

Co-authored-by: Erik Sipsma <erik@sipsma.dev>
Signed-off-by: Justin Chadwell <me@jedevc.com>

solver/llbsolver/ops/exec.go

@@ -318,6 +318,10 @@ func (e *ExecOp) getMountDeps() ([]dep, error) {
 			// if the mount is read-only, then it's also safe, since it can't
 			// be modified by the operation
 			contentBasedCache = true
+		} else if sel == pb.RootMount {
+			// if the mount mounts the entire source, then it's also safe,
+			// since there are no unselected "sneaky" files
+			contentBasedCache = true
 		}
 
 		if m.Dest == pb.RootMount {