docker mysql initdb script can't connect to database

Question

I am using a mysql docker container and added an init script into docker-entrypoint-initdb.d as described here: https://hub.docker.com/_/mysql/ (Initializing a fresh instance).

The init script should use flyway command line tool to migrate the database content. Strangly I get an error that flyway can't connect to the database. Why does this happen? From my understanding the database should be up and running at that point.

[...]
2018-05-18T20:52:45.558031Z 0 [Note] mysqld: ready for connections.
Version: '5.7.22'  socket: '/var/run/mysqld/mysqld.sock'  port: 0  MySQL Community Server (GPL)
Warning: Unable to load '/usr/share/zoneinfo/iso3166.tab' as time zone. Skipping it.
Warning: Unable to load '/usr/share/zoneinfo/leap-seconds.list' as time zone. Skipping it.
Warning: Unable to load '/usr/share/zoneinfo/zone.tab' as time zone. Skipping it.
Warning: Unable to load '/usr/share/zoneinfo/zone1970.tab' as time zone. Skipping it.
2018-05-18T20:52:50.940818Z 4 [Warning] 'user' entry 'root@localhost' ignored in --skip-name-resolve mode.
2018-05-18T20:52:50.940841Z 4 [Warning] 'user' entry 'mysql.session@localhost' ignored in --skip-name-resolve mode.
2018-05-18T20:52:50.940864Z 4 [Warning] 'user' entry 'mysql.sys@localhost' ignored in --skip-name-resolve mode.
2018-05-18T20:52:50.940881Z 4 [Warning] 'db' entry 'performance_schema mysql.session@localhost' ignored in --skip-name-resolve mode.
2018-05-18T20:52:50.940886Z 4 [Warning] 'db' entry 'sys mysql.sys@localhost' ignored in --skip-name-resolve mode.
2018-05-18T20:52:50.940897Z 4 [Warning] 'proxies_priv' entry '@ root@localhost' ignored in --skip-name-resolve mode.
2018-05-18T20:52:50.940926Z 4 [Warning] 'tables_priv' entry 'user mysql.session@localhost' ignored in --skip-name-resolve mode.
2018-05-18T20:52:50.940935Z 4 [Warning] 'tables_priv' entry 'sys_config mysql.sys@localhost' ignored in --skip-name-resolve mode.
mysql: [Warning] Using a password on the command line interface can be insecure.

/usr/local/bin/docker-entrypoint.sh: running /docker-entrypoint-initdb.d/migrate.sh
Flyway Community Edition 5.0.7 by Boxfuse

ERROR: 
Unable to obtain connection from database (jdbc:mysql://localhost/db) for user 'root': Could not connect to address=(host=localhost)(port=3306)(type=master) : Connection refused (Connection refused)
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL State  : 08
Error Code : -1
Message    : Could not connect to address=(host=localhost)(port=3306)(type=master) : Connection refused (Connection refused)

Answer 1

It seems that at the point that the initdb scripts are called MySQL is only available via socket and not via network connection. Therefore flyway can't run. I'm now using second docker container for flyway which conntects to the MySQL container. This works fine.