docker mysql - mis-matched host uid/gid - mysql

Question

I have a mysql docker container that has its data and logs dirs separately mapped to host folders for performance reasons.
I'm using docker-compose to start the container with a group of other related services.
--datadir=/var/lib/mysql/innodb-data
--innodb_log_group_home_dir=/var/lib/mysql/innodb-logs
The container dirs are mapped to the host files system via:
volumes:
- /db/mysql-innodb-data:/var/lib/mysql/innodb-data
- /db/mysql-innodb-logs:/var/lib/mysql/innodb-logs
My problem is that the MySQL container is setting the owner uid to 999.
On the host system this maps to the user 'systemd-coredump'.
Instead I want the container to apply the uid for the hosts 'mysql' user.
I've looked at the MySQL docker container and it has the following logic:
docker_create_db_directories() {
local user; user="$(id -u)"
# TODO other directories that are used by default? like /var/lib/mysql-files
# see https://github.com/docker-library/mysql/issues/562
mkdir -p "$DATADIR"
if [ "$user" = "0" ]; then
# this will cause less disk access than `chown -R`
find "$DATADIR" \! -user mysql -exec chown mysql '{}' +
fi
}
We can see that the above script applies the uid user the container runs under to the data directory. By default the container runs as root.
Given that root is uid 0 I don't actually see how this code is change the data-dirs directory to 999 and as such I suspect this code isn't actually the problem.
So I tried changing the user the container runs as to 'mysql'
mysql:
container_name: mysql
image: mysql:8.0
user: mysql
This changes the container user as expected but then MySQL couldn't start up as there are a number of config files that it can no longer read as it's not running as root.
Here is the full service section from my docker-compose:
mysql:
container_name: mysql
image: mysql:8.0
restart: on-failure
environment:
MYSQL_ROOT_PASSWORD: ${MYSQL_ADMIN_PASSWORD}
MYSQL_DATABASE: ${MYSQL_SCHEMA}
command: >
--user=mysql
--lower-case-table-names=1
--datadir=/var/lib/mysql/innodb-data
--innodb_log_group_home_dir=/var/lib/mysql/innodb-logs
--default-authentication-plugin=mysql_native_password
--max-allowed-packet=512M
--innodb_buffer_pool_instances=${MYSQL_INNODB_BUFFER_POOL_INSTANCES-32}
--innodb_buffer_pool_chunk_size=${MYSQL_INNODB_BUFFER_POOL_CHUNK_SIZE-8M}
--innodb_buffer_pool_size=${MYSQL_INNODB_BUFFER_POOL_SIZE-512M}
--table_open_cache=${MYSQL_TABLE_OPEN_CACHE-512}
--max_connections=${MYSQL_MAX_CONNECTIONS-98}
--innodb_flush_neighbors=0
--innodb_fast_shutdown=2
--innodb_flush_log_at_trx_commit=1
--innodb_flush_method=fsync
--innodb_doublewrite=0
--innodb_use_native_aio=0
--innodb_read_io_threads=10
--innodb_write_io_threads=10
--slow_query_log_file=/tmp/mysql-slow.log --long-query-time=1
--slow_query_log
# mem_limit: ${MYSQL_MEMORY}
volumes:
- /db/mysql-innodb-data:/var/lib/mysql/innodb-data
- /db/mysql-innodb-logs:/var/lib/mysql/innodb-logs
network_mode: "host"
logging:
driver: "journald"