Supported scratch containers in Unified Containerizer.
Supported scratch containers in Unified Containerizer.

Before this patch, we are assuming the 'tmpfs' mount point '/tmp'
always exist in the container's new rootfs. However, this is not
true. For the scratch container (which is a common case), '/tmp'
may not exist, and there may only be an executable binary in the
new rootfs. So we need to create the mount point for 'tmpfs' in
fs::enter().

However, this change may break some cases using the bind backend,
because we are not able the create the '/tmp' mount point in a
read-only filesystem. So we require users to make sure the
directory '/tmp' must already exist in their single layer scratch
images for the following reasons:
  1. For most cases, when operators prefer using the bind backend,
     the single-layer images are usually large in size, and '/tmp'
     exists in the rootfs.
  2. For scratch images, most of them contain more than one layer,
     which means the bind backend cannot be used in those cases.
     So we can create the '/tmp' mount point if it does not exist.
  3. If this is strictly a single layer scratch image, it is
     reasonable that we require users to make sure the mount point
     '/tmp' existed in the image rootfs if they are using the bind
     backend, because we already require the sandbox mount point
     to be existed in those iamges.

Review: https://reviews.apache.org/r/50719/

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