docker-compose tmpfs not working

I have been doing some testing in this regards, it looks like the /var/run directory is special in docker.

Here is some sample config and output:

  ubuntu:
    image: ubuntu
    command: "bash -c 'mount'"
    tmpfs:
      - /var/run
      - /var/cache

Running docker-compose up ubuntu shows what is mounted. Can see /var/cache is mounted but /var/run isn't.

...
ubuntu_1           | tmpfs on /var/cache type tmpfs (rw,nosuid,nodev,noexec,relatime)
...

If you use docker-compose run ubuntu bash you can see it's also mounted there but not /var/run.

The reason is that /var/run is normally a symlink to /run and hence you creating /var/run/mysql as a tmpfs doesn't work.

It will work if you change it to /run/mysql, but /run is normally mounted as tmpfs anyway so you might as well just make /run a tmpfs. Like so:

  ubuntu:
    image: ubuntu
    command: "bash -c 'mount'"
    tmpfs:
      - /run
      - /var/cache

Note: I'd like to amend my answer and show the way to do it using volumes:

services:
  ubuntu:
    image: ubuntu
    command: "bash -c 'mount'"
    volumes:
      - cache_vol:/var/cache
      - run_vol:/run

volumes:
  run_vol:
    driver_opts:
      type: tmpfs
      device: tmpfs
  cache_vol:
    driver_opts:
      type: tmpfs
      device: tmpfs

This also allows you to share the tmpfs mounts if needed.

it some images like alpine the directory /var/run is just a link to /run you can verify that using

$ docker run --rm -ti mariadb:10.1 ls -lh /var/run
lrwxrwxrwx 1 root root 4 Aug  7 13:02 /var/run -> /run

which means that /var/run/mysqld is actually /run/mysqld

your updated docker-compose.yml is

version: '2'
services:
  mysql:
    image: mariadb:10.1
    read_only: true
    tmpfs:
    - /run/mysqld:uid=999,gid=999
    - /tmp
    volumes:
    - mysql:/var/lib/mysql
    restart: always
volumes:
  mysql:

in that case just make your tmpfs point to /run

it looks like the /var/run directory is special in docker.

no, it's just because it's a link