Also, in case of a failure the testing is continued for all SIMD
architectures.

* Dev container added to build and debug the project
* Docker image used for CI is updated to can be used as a base for the
  dev container.
* CI script updated minimally to can be run without root privileges.

Co-authored-by: Michael Platings <michael.platings@arm.com>

Improve test coverage by making try_avoid_tail_loop constexpr

`expected` and `actual` are initialised with different values.

Also refactor dispatch.h

The names of the API functions (marked extern "C") are already prefixed
by the intrinsiccv string.

GCC's shadow declaration check is too sensitive for the library.
As the current OpenCV build turns this check on we need to explicitly
turn it off.