upgrade alpine base image to v3.15.0 including upgrade of openssh-server package to v8.8p1 and rsync package to v3.2.3-r5 (see below)
https://web.archive.org/web/20211204063543/https://www.openssh.com/releasenotes.html
https://git.alpinelinux.org/aports/commit/main/openssh?h=3.15-stable&id=8293eb79d181515827092ec15f9dcb24989959c6
https://git.alpinelinux.org/aports/commit/main/openssh?h=3.15-stable&id=e05faf293fd802c729d432906ffb5e8db77f3830
https://git.alpinelinux.org/aports/commit/main/openssh?h=3.15-stable&id=b5e3d52e5a95ff8a73c23908201474bb15048a11
https://github.com/fphammerle/docker-sftpd/commit/fc472ac4fbc12a25e8831cab197711104923b775
https://git.alpinelinux.org/aports/commit/main/rsync?h=3.15-stable&id=277716a1acbe18a0f2411b2e9b93b8d160204bbf