Docker-compose cannot start mysql, permission denied to /tmp - docker

Question

I'm trying to start a docker container from the vanilla mariadb image from docker hub using docker-compose. While everything previously worked, I'm now getting the error message Can't create/write to file '/tmp/ibLTxiq7' (Errcode: 13 "Permission denied"). Here's the full log:
139707238336448 [Note] /usr/sbin/mysqld (mysqld 10.1.14-MariaDB-1~jessie) starting as process 51 ...
139707238336448 [Note] InnoDB: Using mutexes to ref count buffer pool pages
139707238336448 [Note] InnoDB: The InnoDB memory heap is disabled
139707238336448 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins
139707238336448 [Note] InnoDB: Memory barrier is not used
139707238336448 [Note] InnoDB: Compressed tables use zlib 1.2.8
139707238336448 [Note] InnoDB: Using Linux native AIO
139707238336448 [Note] InnoDB: Using SSE crc32 instructions
139707238336448 [ERROR] mysqld: Can't create/write to file '/tmp/ibLTxiq7' (Errcode: 13 "Permission denied")
7f10205047c0 InnoDB: Error: unable to create temporary file; errno: 13
139707238336448 [ERROR] Plugin 'InnoDB' init function returned error.
139707238336448 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
139707238336448 [ERROR] Unknown/unsupported storage engine: InnoDB
139707238336448 [ERROR] Aborting
I don't understand, does the /tmp folder in the log refer to the host machine or the container? How would I fix this issue?
The command I'm trying to run (as root user) is docker-compose up db. I've checked that there indeed is space on the disk. My OS Ubuntu 16.04.
Update: My docker-compose.yml-file
https-portal:
image: steveltn/https-portal
ports:
- '80:80'
- '443:443'
links:
- wordpress
restart: always
environment:
DOMAINS: 'domain.com -> http://wordpress'
STAGE: 'production'
wordpress:
image: wordpress
links:
- db:mysql
volumes:
- ~/wordpress/wp_html:/var/www/html
db:
image: mariadb
environment:
MYSQL_ROOT_PASSWORD: [---]
Update 2:
I seem to have (at least temporarily) solved the problem, rather surprisingly, by adding a volume mount path for the db container:
db:
image: mariadb
volumes:
- /data/maridb:/var/lib/mysql
However this does not actually affect anything, it just prints a warning that the volume mount is not used (and indeed it isn't) because the container has previously been created with a different volume and the old one is used. Still, the original problem disappears. Any thoughts on what causes this behavior?