I have observed the same thing with my setup.
Below is what I have observed:
1. Just right after starting the CrashPlan docker, cache is still pointing to /config/cache
lrwxrwxrwx 1 root root 11 Sep 10 09:08 bin -> /config/bin
lrwxrwxrwx 1 root root 13 Sep 10 09:08 cache -> /config/cache
lrwxrwxrwx 1 root root 12 Sep 10 09:08 conf -> /config/conf
drwxrwxrwx 1 nobody root 82 Apr 29 21:22 doc
2. After a few seconds, cache is not pointing to /config/cache anymore
lrwxrwxrwx 1 root root 11 Sep 10 09:08 bin -> /config/bin
drwxrwxrwx 1 root root 578 Sep 10 09:09 cache
lrwxrwxrwx 1 root root 12 Sep 10 09:08 conf -> /config/conf
drwxrwxr-x 1 root root 82 Sep 10 09:09 doc
Because of this, everytime I restart my CrashPlan docker, it would scan files and synchronise. Has anyone resolved this? Thanks