Support docker secrets for WORDPRESS_DB_PASSWORD

[SKoschnicke]

The official MySQL docker image supports docker secrets to set the database password. This seems not to be possible with this image, or am I missing something?

If it is not yet available, I would implement it parallel to the MySQL image functionality (support for WORDPRESS_DB_PASSWORD_FILE)

[yosifkit]

Those should already exist: #186

[SKoschnicke]

You are right, thank you! How about a PR for adding this to the readme? :)

[tianon]

That'd be great! The appropriate file for that would be https://github.com/docker-library/docs/blob/master/wordpress/content.md. :+1:

[SKoschnicke]

Sorry for the delay, added it.