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

manage.sh

@@ -158,6 +158,74 @@ grunt_build() {
     grunt --gruntfile "$SEARX_DIR/static/themes/simple/gruntfile.js"
 }
 
+docker_build() {
+    # Check if it is a git repository
+    if [ ! -d .git ]; then
+	echo "This is not Git repository"
+	exit 1
+    fi
+
+    if [ ! -x "$(which git)" ]; then
+	echo "git is not installed"
+	exit 1
+    fi
+
+    if [ ! git remote get-url origin 2> /dev/null ]; then
+	echo "there is no remote origin"
+	exit 1
+    fi
+
+    # This is a git repository
+
+    # "git describe" to get the Docker version (for example : v0.15.0-89-g0585788e)
+    # awk to remove the "v" and the "g"
+    SEARX_GIT_VERSION=$(git describe --match "v[0-9]*\.[0-9]*\.[0-9]*" HEAD 2>/dev/null | awk -F'-' '{OFS="-"; $1=substr($1, 2); $3=substr($3, 2); print}')
+
+    # add the suffix "-dirty" if the repository has uncommited change
+    git update-index -q --refresh
+    if [ ! -z "$(git diff-index --name-only HEAD --)" ]; then
+	SEARX_GIT_VERSION="${SEARX_GIT_VERSION}-dirty"
+    fi
+
+    # Get the last git commit id, will be added to the Searx version (see Dockerfile)
+    VERSION_GITCOMMIT=$(echo $SEARX_GIT_VERSION | cut -d- -f2-4)
+    echo "Last commit : $VERSION_GITCOMMIT"
+
+    # Check consistency between the git tag and the searx/version.py file
+    # /!\ HACK : parse Python file with bash /!\
+    # otherwise it is not possible build the docker image without all Python dependencies ( version.py loads __init__.py )
+    # SEARX_PYTHON_VERSION=$(python -c "import six; import searx.version; six.print_(searx.version.VERSION_STRING)")
+    SEARX_PYTHON_VERSION=$(cat searx/version.py | grep "\(VERSION_MAJOR\|VERSION_MINOR\|VERSION_BUILD\) =" | cut -d\= -f2 | sed -e 's/^[[:space:]]*//' | paste -sd "." -)
+    if [ $(echo "$SEARX_GIT_VERSION" | cut -d- -f1) != "$SEARX_PYTHON_VERSION" ]; then
+	echo "Inconsistency between the last git tag and the searx/version.py file"
+	echo "git tag:          $SEARX_GIT_VERSION"
+	echo "searx/version.py: $SEARX_PYTHON_VERSION"
+	exit 1
+    fi
+
+    # define the docker image name
+    # /!\ HACK to get the user name /!\
+    GITHUB_USER=$(git remote get-url origin | sed 's/.*github\.com\/\([^\/]*\).*/\1/')
+    SEARX_IMAGE_NAME="${GITHUB_USER:-searx}/searx"
+
+    # build Docker image
+    echo "Building image ${SEARX_IMAGE_NAME}:${SEARX_GIT_VERSION}"
+    sudo docker build \
+         --build-arg SEARX_GIT_VERSION="${SEARX_GIT_VERSION}" \
+         --build-arg VERSION_GITCOMMIT="${VERSION_GITCOMMIT}" \
+         --build-arg LABEL_DATE=$(date -u +"%Y-%m-%dT%H:%M:%SZ") \
+         --build-arg LABEL_VCS_REF=$(git rev-parse HEAD) \
+         --build-arg LABEL_VCS_URL=$(git remote get-url origin) \
+	 --build-arg TIMESTAMP_SETTINGS=$(git log -1 --format="%cd" --date=unix -- searx/settings.yml) \
+	 --build-arg TIMESTAMP_UWSGI=$(git log -1 --format="%cd" --date=unix -- dockerfiles/uwsgi.ini) \
+         -t ${SEARX_IMAGE_NAME}:latest -t ${SEARX_IMAGE_NAME}:${SEARX_GIT_VERSION} .
+
+    if [ "$1" = "push" ]; then
+	sudo docker push ${SEARX_IMAGE_NAME}:latest
+	sudo docker push ${SEARX_IMAGE_NAME}:${SEARX_GIT_VERSION}
+    fi
+}
+
 #
 # Help
 #
@@ -182,6 +250,7 @@ Commands
     locales              - Compile locales
     styles               - Build less files
     grunt_build          - Build files for themes
+    docker_build         - Build Docker image
 
     Tests
     -----