Over at the Other Place there's a post that details how to install Docker binaries on a DS220j, being an ARM 7 32-bit device. It seems an interesting prospect and I see @Telos is already aware. My concern when looking at the extract tarball of binaries is that they all are assigned 'rwxrwxrwx' my admin user and group 'users': I'd expect these to be somewhat more restricted.
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Docker on ARM-based Synology DS-line devices
How to run Docker on ARM-based Synology DS-line NAS