Doc: Add "Init DB" section (#4695)

Misskey 11.x requires database initialization.

docs/docker.en.md

@@ -57,7 +57,13 @@ Build misskey with the following:
 
 `docker-compose build`
 
-*5.* That is it.
+*5.* Init DB
+----------------------------------------------------------------
+``` shell
+docker-compose run --rm web npm run init
+```
+
+*6.* That is it.
 ----------------------------------------------------------------
 Well done! Now you have an environment to run Misskey.
 

docs/docker.ja.md

@@ -57,7 +57,13 @@ cp docker_example.env docker.env
 
 `docker-compose build`
 
-*5.* 以上です！
+*5.* データベースを初期化
+----------------------------------------------------------------
+``` shell
+docker-compose run --rm web npm run init
+```
+
+*6.* 以上です！
 ----------------------------------------------------------------
 お疲れ様でした。これでMisskeyを動かす準備は整いました。