Containers parked in public spaces must be marked and secured in accordance with § 32 StVO (German Road Traffic Regulations), as they constitute an obstacle to traffic (obligation to ensure traffic safety).
§ 32 StVO
(1) It is prohibited to soil or wet the road or to bring objects onto roads or leave them there if this may endanger or impede traffic. Whoever is responsible for such conditions contrary to traffic regulations must remove them immediately and in the meantime make them sufficiently recognisable. If necessary (§ 17 paragraph 1), traffic obstructions shall be illuminated with their own light source or made visible by other approved lighting equipment.
Quelle: https://www.gesetze-im-internet.de/stvo_2013/__32.html
How are containers marked with warning markings in accordance with the regulations?
In the german “Verkehrsblatt” (VkBl, Official Gazette of the Federal Ministry of Transport) of 28.04. 1982, page 186, the following regulations are laid down for the marking of containers:
- Containers up to a width of 2.5 m and a length of 8 m are to be marked within built-up areas by a retro-reflective warning marking according to DIN 67520 (min. reflection class RA2).
- A total of 8 warning strips with the prescribed dimensions 141 x 705 mm are required for marking a container or swap body. These are to be red and white (left and right facing) and firmly attached to the container in accordance with DIN 6171 6.
- Two warning strips are to be attached vertically to the outermost edge of each side and end face, not deeper than 0.40 m and not higher than 1.55 m. If the available space is not sufficient, they may also be affixed horizontally.
The application of 
Container warning markings | reduces the potential danger and accidents caused by unmarked or poorly marked containers can thus be avoided. They are always applied in such a way that a roof / A is formed in the centre and the rhombs point downwards from the centre to the outside:
