Version 5.7.0

Changes:

• New command search: find images on Docker Hub.
• New option for clear-cache: pass --orphan to remove only blobs without references.
• Support zstd compression for backup, restore and install - only if host Python has compression.zstd.
• Verify content of cached layer blobs.
• Layer whiteout names (.wh..., .wh., .wh..) and trailing-. member names are now filtered like ordinary, closing an escape that emptied the container directory during a plain install.
• Use file descriptors with O_NOFOLLOW for /etc fixups.
• Normalize paths for proot-distro build Dockerfile's WORKDIR, COPY and ADD commands.