Add documentation for existing redmine env vars:

- `REDMINE_DB_MYSQL`, `REDMINE_DB_POSTGRES`, `REDMINE_DB_PORT`, `REDMINE_DB_USERNAME`, `REDMINE_DB_PASSWORD`, `REDMINE_DB_DATABASE`, `REDMINE_DB_ENCODING`, and `REDMINE_PLUGINS_MIGRATE`

redmine/content.md

@@ -86,12 +86,52 @@ If you'd like to be able to access the instance from the host without the contai
 
 When you start the `%%REPO%%` image, you can adjust the configuration of the instance by passing one or more environment variables on the `docker run` command line.
 
+### `REDMINE_DB_MYSQL` or `REDMINE_DB_POSTGRES`
+
+These two variables allow you to set the hostname or IP address of the MySQL or PostgreSQL host, respectively. These values are mutually exclusive so it is undefined behavior if both are set. If neither variable is set, the image will fall back to using SQLite.
+
+### `REDMINE_DB_PORT`
+
+This variable allows you to specify a custom database connection port. If unspecified, it will default to the regular connection ports: 3306 for MySQL, 5432 for PostgreSQL, and empty string for SQLite.
+
+### `REDMINE_DB_USERNAME`
+
+This variable sets the user that Redmine and any rake tasks use to connect to the specified database. If unspecified, it will default to `root` for MySQL, `postgres` for PostgreSQL, or `redmine` for SQLite.
+
+### `REDMINE_DB_PASSWORD`
+
+This variable sets the password that the specified user will use in connecting to the database. There is no default value.
+
+### `REDMINE_DB_DATABASE`
+
+This variable sets the database that Redmine will use in the specified database server. If not specified, it will default to `redmine` for MySQL, the value of `REDMINE_DB_USERNAME` for PostgreSQL, or `sqlite/redmine.db` for SQLite.
+
+### `REDMINE_DB_ENCODING`
+
+This variable sets the character encoding to use when connecting to the database server. If unspecified, it will use the default for the `mysql2` library ([`UTF-8`](https://github.com/brianmario/mysql2/tree/18673e8d8663a56213a980212e1092c2220faa92#mysql2---a-modern-simple-and-very-fast-mysql-library-for-ruby---binding-to-libmysql)) for MySQL, `utf8` for PostgreSQL, or `utf8` for SQLite.
+
 ### `REDMINE_NO_DB_MIGRATE`
 
 This variable allows you to control if `rake db:migrate` is run on container start. Just set the variable to a non-empty string like `1` or `true` and the migrate script will not automatically run on container start.
 
 `db:migrate` will also not run if you start your image with something other than the default `CMD`, like `bash`. See the current `docker-entrypoint.sh` in your image for details.
 
+### `REDMINE_PLUGINS_MIGRATE`
+
+This variable allows you to control if `rake redmine:plugins:migrate` is run on container start. Just set the variable to a non-empty string like `1` or `true` and the migrate script will be automatically run on every container start. It will be run after `db:migrate`.
+
+`redmine:plugins:migrate` will not run if you start your image with something other than the default `CMD`, like `bash`. See the current `docker-entrypoint.sh` in your image for details.
+
 ### `REDMINE_SECRET_KEY_BASE`
 
 This variable is used to create an initial `config/secrets.yml` and set the `secret_key_base` value, which is "used by Rails to encode cookies storing session data thus preventing their tampering. Generating a new secret token invalidates all existing sessions after restart" ([session store](https://www.redmine.org/projects/redmine/wiki/RedmineInstall#Step-5-Session-store-secret-generation)). If you do not set this variable or provide a `secrets.yml` one will be generated using `rake generate_secret_token`.
+
+## Docker Secrets
+
+As an alternative to passing sensitive information via environment variables, `_FILE` may be appended to the previously listed environment variables, causing the initialization script to load the values for those variables from files present in the container. In particular, this can be used to load passwords from Docker secrets stored in `/run/secrets/<secret_name>` files. For example:
+
+```console
+$ docker run -d --name some-%%REPO%% -e REDMINE_DB_MYSQL_FILE=/run/secrets/mysql-host -e REDMINE_DB_PASSWORD_FILE=/run/secrets/mysql-root %%REPO%%:tag
+```
+
+Currently, this is only supported for `REDMINE_DB_MYSQL`, `REDMINE_DB_POSTGRES`, `REDMINE_DB_PORT`, `REDMINE_DB_USERNAME`, `REDMINE_DB_PASSWORD`, `REDMINE_DB_DATABASE`, `REDMINE_DB_ENCODING`, and `REDMINE_SECRET_KEY_BASE`.