Update Docker image

See #1561 , use uwsgi and Alpine Linux

Volume:
/var/log/uwsgi contains error log for 2 days (file uwsgi.log)
/etc/searx contains the settings.yml and uwsgi.ini files.
The docker image creates them if they don't exist.
The two files can be modified after the first run. See below.

Environement variables:
MORTY_URL : external URL of Morty
MORTY_KEY : base64 encoded key
BASE_URL : external URL of Searx
BIND_ADDRESS : internal HTTP port to listen to

Labels : org.label-schema.schema.*

Parameters:
-h : display this help
-d : will update the settings and quit immediately (settings.yml and uwsgi.ini)
-f : always update the settings (previous version saved with suffix .old).
     without this parameter, the new settings are copied with suffix .new

When the Docker image contains newer settings:
- without -f parameter: the new versions are copied to /etc/searx/settings.yml.new and /etc/searx/uwsgi.ini.new.
- with -f parameter:  the old versions are renamed with .old suffix. The new version replaces /etc/searx/settings.yml and /etc/searx/uwsgi.ini

Build using "./manage.sh docker_build", add "push" as parameter also push the Docker image.
The script requires a git repository to work (it makes sure that the last git tag matches searx/version.py)
"git describe" is used to create a meaningful version.
Example : 0.15.0-90-49c5bcb4-dirty (dirty means that the docker image was made with uncommited changes).

Use "docker inspect -f {{.Config.Labels.version}} searx" to get the version of an existing image.

.dockerignore based on .gitignore

.travis.yml: include docker stage

doi_resolvers / default_doi_resolver were missing in the settings_robots.yml file, so the test server was not able to start (crash). Since the output wasn't displayed, it was not obvious why the Selenium couldn't connect to searx.

Improve infinite scroll (and with it fixing a bug)

fix: dev: python3 dict keys jsonify

Add addr setup via SEARX_BIND_ADDRESS

This improves the user experience by loading in the next entries shortly before him getting to the bottom. It makes the scrolling more smooth without a break in between.

It also fixes an error on my browser that scrolling never hits the defined number. When I debugged it I hit `.scrolltop` of 1092.5 and the `doc.height - win.height` of 1093, so the condition was never true.

install coverage_plugin of nose2 instead of coverage-plugin

google_images :  use JSON embedded in HTML (engine expected pure JSON)

Remove some engines : subtitleseeker, seedpeer, swisscows

http://www.subtitleseeker.com and http://www.seedpeer.eu don't exist anymore.
https://swisscows.ch/ has change : the engine needs to be updated

[fix] seznam engine

requirements.txt : update dependencies

simple theme : use instance_name for page title

Fix google image search engine

- Because there is not full image url in the dom, we replace "image_url" with the same url as the "url" (url of source).
  See example HTML https://gist.github.com/Nachtalb/2dea8a4d2c723c49226ad9645838121f
- Remove unused import
- Fix google image search title
- Keep google image safe value up to date

[fix] DuckDuckGo Images requests

related to #1025

Use HTTPS for crossref