Add override default specification

[niccokunzmann]

Docker compose allows adding override specifications.
This is useful if an installation would like to adapt the default specification but only a few values of it.

default_specification.override.yml
DEFAULT_SPECIFICATION=path/to/default_specification.override.yml

The override values should be added on top of the default specification but be lower in priority than the in-app specification dict.

See also:

• Multiple specification_urls #323

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