Docker import mysql database

docker exec -i <container_id> mysql -uroot -p<root_pass> <dbname> < mydump.sql

or if you are dealing with gunzip and mariadb

gunzip -c my_db_dump.sql.gz \
| sed '1{/sandbox mode/d;}' \
| docker exec -i container_id \
mariadb --binary-mode=1 -uroot -pRootpassword targetdb