[enh] add engine - Docker Hub

Slightly modified merge of commit [1cb1d3ac] from searx [PR 2543]:

      This adds Docker Hub .. as a search engine .. the engine's favicon was
      downloaded from the Docker Hub website with wget and converted to a PNG
      with ImageMagick .. It supports the parsing of URLs, titles, content,
      published dates, and thumbnails of Docker images.

[1cb1d3ac] https://github.com/searx/searx/pull/2543/commits/1cb1d3ac
[PR 2543] https://github.com/searx/searx/pull/2543



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>