Joint docker image data location

[LarssonOliver]

Describe the bug
According to the quickstart guide, the default data location for docker images is

# ─── Database ─────────────────────────────────────────────
# Docker (default):     /app/data/invio.db
# Local development:    ./invio.db
DATABASE_PATH=/app/data/invio.db

However, in reality in the joint image, the db is placed at /app/backend/invio.db.

Similarly, logos are located in /app/backend/logos and templates in /app/backend/static/templates, perhaps it would make sense to have a DATA_DIR variable or similar where all such files could be gathered?

Example:

/data/invio.db
/data/logos
/data/templates
etc...

Platform you host on
Docker, image ghcr.io/kittendevv/invio:v2.0.2