It seems that server 2.7.4 introduced a change that did not work with the methods florider and etho201 use to create the ARM versions. etho201 has fixed the issue with their ARM build process.
In this post etho201 explains.
The same thread also has an entry about email. Email is set up using your .env file with all the other Joplin server variables.
It should. Could it be that the amount of data (1MB) is so small that it gets rounded down to zero. Does the sync actually work?
I am not sure what the "logs" section is the GUI is for. I cannot recall ever seeing anything in there. The "items" section used to show the files on the server but I notice that this (and "logs") has been removed from user accounts and only remains available to the admin. I do not use the admin account for syncing so there is no data and I cannot check for you.
docker logs joplin_app_1 will show the log for the Joplin server (assuming your Joplin container is joplin_app_1).
docker logs joplin_db_1 will show a log for the database (assuming your PostGres container is joplin_db_1).
I have no idea and so will leave this question for experts such as @MrKanister 