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

.travis.yml

@@ -1,16 +1,14 @@
+language: python
 sudo: false
 cache:
   - pip
   - npm
   - directories:
     - $HOME/.cache/pip
+
 addons:
   firefox: "latest"
-language: python
-python:
-  - "2.7"
-  - "3.5"
-  - "3.6"
+
 before_install:
   - "export DISPLAY=:99.0"
   - "sh -e /etc/init.d/xvfb start"
@@ -27,6 +25,31 @@ script:
 after_success:
   - ./manage.sh py_test_coverage
   - codecov
+
+stages:
+  - test
+  - name: docker
+    if: branch = master AND type != pull_request AND env(DOCKER_USERNAME) IS present
+
+jobs:
+  include:
+    - python: "2.7"
+    - python: "3.5"
+    - python: "3.6"
+    - stage: docker
+      python: "3.6"
+      git:
+        depth: false
+      services:
+        - docker
+      addons: []
+      before_install: true
+      install: true
+      script:
+        - echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin
+        - ./manage.sh docker_build push
+      after_success: true
+
 notifications:
   irc:
     channels: