Allow overriding instance directory location

docs/installation.rst

@@ -150,10 +150,12 @@ Configuration
 -------------
 
 Configuration and runtime data is stored in the instance directory.  For
-container installs, this is ``/usr/local/var/schilder2000-instance/``.
-For package installs, this is ``{{ python prefix }}/var/schilder2000-instance``;
-if in doubt, try to run ``flask -A schilder2000``, the error should tell you
-where it expects the instance directory.
+container installs, this is ``/usr/local/var/schilder2000-instance/``.  For
+package installs, this defaults to ``{{ python prefix
+}}/var/schilder2000-instance``; if in doubt, try to run ``flask -A
+schilder2000``, the error should tell you where it expects the instance
+directory.  To override, specify an *absolute* path in the
+``SCHILDER2000_INSTANCE_PATH`` environment variable.
 
 Example config and data is located in the ``examples`` directory.  The templates
 there get their footer text and logo from the application config and should also

schilder2000/__init__.py

+import os
 from pathlib import Path
 
 from flask_multipass import Multipass
@@ -15,7 +16,11 @@ multipass = Multipass()
 
 
 def create_app():
-    app = Flask(__name__, instance_relative_config=True)
+    app = Flask(
+        __name__,
+        instance_path=os.getenv("SCHILDER2000_INSTANCE_PATH"),
+        instance_relative_config=True,
+    )
     app.config.from_pyfile("config/config.py")
 
     app.cli.add_command(cli.alembic)