switch continuous integration system to Drone CI:

- add .drone.yml, reimplement tasks from .travis.yml (php/node/python test suites)
- ref. https://github.com/shaarli/Shaarli/issues/1649
- https://www.drone.io/enterprise/opensource/
- https://cloud.drone.io/shaarli/Shaarli/settings
- https://docs.drone.io/pipeline/docker/syntax/steps/
- https://docs.drone.io/pipeline/docker/syntax/cloning/
- https://docs.drone.io/pipeline/environment/reference/
- https://docs.drone.io/pipeline/configuration/
- https://docs.drone.io/pipeline/overview/
- https://docs.drone.io/pipeline/docker/syntax/volumes/
- https://docs.drone.io/pipeline/docker/syntax/volumes/temporary/
- php: use custom php base images with prebuilt PHP extensions and composer (speed up build process) (https://hub.docker.com/r/shaarli/drone-ci-base, https://www.github.com/shaarli/drone-ci-base, https://hub.docker.com/repository/docker/shaarli/drone-ci-base/builds)
- no need to port $PATH update command, the default node:10 image already has the correct $PATH
- don't force pull of latest images to speed up builds
- the default is to use a local/cached image when the image is not tagged :latest
- https://docs.drone.io/pipeline/docker/syntax/parallelism/
- https://docs.drone.io/pipeline/environment/reference/drone-git-http-url/
- ignore PHP version requirements for https://packagist.org/packages/pubsubhubbub/publisher, pending merge of https://github.com/pubsubhubbub/php-publisher/pull/11
- closes https://github.com/shaarli/Shaarli/issues/1649

.drone.yml

@@ -0,0 +1,115 @@
+---
+
+kind: pipeline
+type: docker
+name: php-8.0
+
+steps:
+  - name: test-php-8.0
+    image: shaarli/drone-ci-base:8.0
+    commands:
+      - composer config --unset platform && composer config platform.php 8.0
+      - composer update --ignore-platform-req=php
+      - composer remove --dev --ignore-platform-req=php phpunit/phpunit
+      - composer require --dev --ignore-platform-req=php phpunit/php-text-template ^2.0
+      - composer require --dev --ignore-platform-req=php phpunit/phpunit ^9.0
+      - make clean
+      - make check_permissions
+      - make code_sniffer
+      - make all_tests
+
+---
+
+kind: pipeline
+type: docker
+name: php-7.4
+
+steps:
+  - name: test-php-7.4
+    image: shaarli/drone-ci-base:7.4
+    commands:
+      - composer config --unset platform && composer config platform.php 7.4
+      - composer update
+      - make clean
+      - make check_permissions
+      - make code_sniffer
+      - make all_tests
+
+---
+
+kind: pipeline
+type: docker
+name: php-7.3
+
+steps:
+  - name: test-php-7.3
+    image: shaarli/drone-ci-base:7.3
+    commands:
+      - composer config --unset platform && composer config platform.php 7.3
+      - composer update
+      - make clean
+      - make check_permissions
+      - make code_sniffer
+      - make all_tests
+
+---
+
+kind: pipeline
+type: docker
+name: php-7.2
+
+steps:
+  - name: test-php-7.2
+    image: shaarli/drone-ci-base:7.2
+    commands:
+      - composer config --unset platform && composer config platform.php 7.2
+      - composer update
+      - make clean
+      - make check_permissions
+      - make code_sniffer
+      - make all_tests
+
+---
+
+kind: pipeline
+type: docker
+name: php-7.1
+
+steps:
+  - name: test-php-7.1
+    image: shaarli/drone-ci-base:7.1
+    commands:
+      - composer config --unset platform && composer config platform.php 7.1
+      - composer update
+      - make clean
+      - make check_permissions
+      - make code_sniffer
+      - make all_tests
+
+---
+
+kind: pipeline
+type: docker
+name: node-10
+
+steps:
+  - name: test-node-10
+    image: node:10
+    commands:
+      - yarn install
+      - yarn run build # verify successful frontend builds
+      - make eslint # javascript static analysis
+      - make sasslint # linter for SASS syntax
+
+---
+
+kind: pipeline
+type: docker
+name: python-3.6
+
+steps:
+  - name: test-python-3.6
+    image: python:3.6-slim
+    commands:
+      - pip install mkdocs
+      - mkdocs build --clean

.travis.yml

@@ -1,68 +0,0 @@
-dist: bionic
-
-matrix:
-  include:
-    # jobs for each supported php version
-    - language: php
-      php: nightly # PHP 8.0
-      install:
-        - composer self-update --2
-        - composer update --ignore-platform-req=php
-        - composer remove --dev --ignore-platform-req=php phpunit/phpunit
-        - composer require --dev --ignore-platform-req=php phpunit/php-text-template ^2.0
-        - composer require --dev --ignore-platform-req=php phpunit/phpunit ^9.0
-    - language: php
-      php: 7.4
-    - language: php
-      php: 7.3
-    - language: php
-      php: 7.2
-    - language: php
-      php: 7.1
-    # jobs for frontend builds
-    - language: node_js
-      node_js: 10
-      cache:
-        yarn: true
-        directories:
-          - $HOME/.cache/yarn
-      install:
-        - yarn install
-      before_script:
-        - PATH=${PATH//:\.\/node_modules\/\.bin/}
-      script:
-        - yarn run build # verify successful frontend builds
-        - make eslint # javascript static analysis
-        - make sasslint # linter for SASS syntax
-    # jobs for documentation builds
-    - language: python
-      python: 3.6
-      cache:
-        directories:
-          - $HOME/.cache/pip
-      install:
-        - pip install mkdocs
-      script:
-        - mkdocs build --clean
-
-cache:
-  directories:
-    - $HOME/.composer/cache
-
-before_install:
-  # Disable xdebug: it significantly speed up tests and linter, and we don't use coverage yet
-  - phpenv config-rm xdebug.ini || echo 'No xdebug config.'
-
-install:
-  # install/update composer and php dependencies
-  - composer config --unset platform && composer config platform.php $TRAVIS_PHP_VERSION
-  - composer update
-
-before_script:
-  - PATH=${PATH//:\.\/node_modules\/\.bin/}
-
-script:
-  - make clean
-  - make check_permissions
-  - make code_sniffer
-  - make all_tests