Add a guide on how to migrate database

docs/move-existing-database.md

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+# Move an existing ./app.db database
+
+If you are coming from the original version of BrickTracker and you wish to move your database from a bind mount to a volume, you can follow this procedure.
+
+> **Note**<br>
+> If you already have a copy of your `app.db` file easily available, you can skip directly to "Move the database" section
+
+## Update to version 1.0.0
+
+Follow the release instructions to upgrade your application to version `1.0.0`.
+
+**However**, make sure you unset the `BK_DATABASE_PATH` variable, or set it with the value `./app.db`. For instance:
+
+```
+services:
+  bricktracker:
+    container_name: BrickTracker
+    restart: unless-stopped
+    image: gitea.baerentsen.space/frederikbaerentsen/bricktracker:1.0.0
+    ports:
+      - "3333:3333"
+    volumes:
+      - ./app.db:/app/app.db
+      - data:/data/
+      - instructions:/app/static/instructions/
+      - minifigures:/app/static/minifigures/
+      - parts:/app/static/parts/
+      - sets:/app/static/sets/
+    # Or define those in your .env file
+    environment:
+      #BK_DATABASE_PATH: /data/app.db
+      BK_MINIFIGURES_FOLDER: minifigures
+      BK_RETIRED_SETS_PATH: /data/retired_sets.csv
+      BK_THEMES_PATH: /data/themes.csv
+    #env_file: ".env"
+
+volumes:
+  data:
+  instructions:
+  minifigures:
+  parts:
+  sets:
+```
+
+Redeploy your application to version `1.0.0`
+
+## Download your database file
+
+You should see the new version of the application running.
+Go to the **Admin** page, and in the **Database** section press the **Download the database file** button.
+This will download a copy of the database file to your computer.
+
+![](images/move-existing-database-01.png)
+
+## Move the database
+
+Now that you have a copy of your database, you can tell the application its new path.
+Edit the `BK_DATABASE_PATH` environment variable and redeploy your application.
+
+> **Note**<br>
+> The name of the database file does not have to be `app.db`
+
+For instance:
+
+```
+services:
+  bricktracker:
+    container_name: BrickTracker
+    restart: unless-stopped
+    image: gitea.baerentsen.space/frederikbaerentsen/bricktracker:1.0.0
+    ports:
+      - "3333:3333"
+    volumes:
+      - data:/data/
+      - instructions:/app/static/instructions/
+      - minifigures:/app/static/minifigures/
+      - parts:/app/static/parts/
+      - sets:/app/static/sets/
+    # Or define those in your .env file
+    environment:
+      BK_DATABASE_PATH: /data/bricktracker.db
+      BK_MINIFIGURES_FOLDER: minifigures
+      BK_RETIRED_SETS_PATH: /data/retired_sets.csv
+      BK_THEMES_PATH: /data/themes.csv
+    #env_file: ".env"
+
+volumes:
+  data:
+  instructions:
+  minifigures:
+  parts:
+  sets:
+```
+
+Redeploy your application.
+
+## Upload the database
+
+When you reload the page of your application, you will be greeted with an error message.
+This is expected as you have told the application the database has moved somewhere else.
+
+![](images/move-existing-database-02.png)
+
+Navigate the **Admin** page once again.
+You should see that the path has changed and the database is not initialized.
+
+![](images/move-existing-database-05.png)
+
+This time open the **Database danger zone**.
+
+![](images/move-existing-database-03.png)
+
+Press the **Import a database file** file and select the file you downloaded earlier.
+
+![](images/move-existing-database-04.png)
+
+Press the **Import a database file** button.
+
+![](images/move-existing-database-06.png)
+
+Done. Your records should be back.
+
+## Update other files
+
+While you are at it, check the following sections to make sure everything is ok:
+
+- Default images
+- Themes
+- Retired sets