Use init container for database initialization

[andrenarchy]

Currently, initializeDatabase passes down INITIALIZE_DATABASE=true to the Stellar Core container. Instead, we should use an init container for this operation instead of using the Stellar Core container's entrypoint.

Note: I think we should make sure that we don't overwrite an existing database, so the logic from https://github.com/satoshipay/docker-stellar-core/blob/8dac8b3df22f1e1ec92cde6f09caf3b54199b8c6/entry.sh#L5-L25 should be copied.