refactor: use .env for Authelia password hash

- Rename users_database.yml to users_database.template.yml
- Use envsubst to substitute AUTHELIA_USER_PASSWORD_HASH from .env
- Update configuration.yml to use /config/users_database.yml
- Add AUTHELIA_USER_PASSWORD_HASH environment variable to compose
- Password hash now stored securely in .env instead of git

net/authelia/users_database.template.yml

@@ -0,0 +1,16 @@
+---
+###############################################################
+#                         Users Database                       #
+###############################################################
+
+# This file can be used if you do not have an LDAP set up.
+
+# List of users
+users:
+  valknar:
+    displayname: "Valknar"
+    password: "${AUTHELIA_USER_PASSWORD_HASH}"
+    email: valknar@pivoine.art
+    groups:
+      - admins
+      - dev