Previous check based on dest attributes was not correct
as the attributes already get converted before validation happens.

Because the local path is not preserved for single-file
outputs and gets replaced by io.Writer, a temporary array variable
was needed. This value should instead be added to ExportEntry
struct in BuildKit in future revision.

Signed-off-by: Tonis Tiigi <[email protected]>

If override specifies a path, mark it automatically allowed
so there is no need to use duplicate flags for defining the
same feature.

Signed-off-by: Tonis Tiigi <[email protected]>