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

dockerfiles/uwsgi.ini

@@ -0,0 +1,33 @@
+[uwsgi]
+# Who will run the code
+uid = searx
+gid = searx
+
+# Number of workers (usually CPU count)
+workers = 4
+
+# The right granted on the created socket
+chmod-socket = 666
+
+# Plugin to use and interpretor config
+single-interpreter = true
+master = true
+plugin = python3
+lazy-apps = true
+enable-threads = true
+
+# Module to import
+module = searx.webapp
+
+# Virtualenv and python path
+pythonpath = /usr/local/searx/
+chdir = /usr/local/searx/searx/
+
+# Disable logging for privacy
+disable-logging=True
+
+# But keep errors for 2 days
+touch-logrotate = /run/uwsgi-logrotate
+unique-cron = 15 0 -1 -1 -1 { touch /run/uwsgi-logrotate  }
+log-backupname = /var/log/uwsgi/uwsgi.log.1
+logto = /var/log/uwsgi/uwsgi.log