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

.dockerignore

@@ -0,0 +1,41 @@
+*~
+*/*~
+*/*/*~
+*/*/*/*~
+*/*/*/*/*~
+
+# Git
+.git
+.gitignore
+
+# CI
+.codeclimate.yml
+.travis.yml
+.taskcluster.yml
+
+# Byte-compiled / optimized / DLL files
+__pycache__/
+*/__pycache__/
+*/*/__pycache__/
+*/*/*/__pycache__/
+*.py[cod]
+*/*.py[cod]
+*/*/*.py[cod]
+*/*/*/*.py[cod]
+
+# to sync with .gitignore
+.coverage
+coverage/
+.installed.cfg
+engines.cfg
+env
+searx-ve
+robot_log.html
+robot_output.xml
+robot_report.html
+test_basic/
+setup.cfg
+
+node_modules/
+
+.tx/