WIP: switch continuous integration system to Drone CI

- add .drone.yml, reimplement tasks from .travis.yml
- ref. shaarli#1649
- https://www.drone.io/enterprise/opensource/
- https://cloud.drone.io/nodiscc/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/
- use custom php 7.4, 7.3 images with prebuilt PHP extensions and composer (speed up build process) (https://hub.docker.com/r/nodiscc/drone-ci-base, https://www.github.com/nodiscc/drone-ci-base, https://docs.docker.com/docker-hub/builds/)
- disable builtin drone clone action since it only supports cloning as root (drone/drone-git#23, https://docs.drone.io/pipeline/docker/syntax/cloning/)
- drone: disable updating $PATH for node toolchain as it results in an empty $PATH

.drone.yml

@@ -0,0 +1,61 @@
+---
+kind: pipeline
+type: docker
+name: php-7.4
+
+clone:
+  disable: true
+
+steps:
+- name: test-php-7.4
+  image: nodiscc/drone-ci-base:7.4
+  commands:
+    - whoami
+    - id
+    - echo $PWD
+    - ls -ld $PWD
+    - ls -ld /drone
+    - echo $HOME
+    - git clone https://github.com/nodiscc/Shaarli.git .
+    - git checkout $DRONE_COMMIT
+    - phpenv config-rm xdebug.ini || echo 'No xdebug config.'
+    # install/update composer and php dependencies
+    - composer config --unset platform && composer config platform.php 7.4
+    - composer update
+    # - PATH=${PATH//:\.\/node_modules\/\.bin/}
+    # run tests
+    - make clean
+    - make check_permissions
+    - make code_sniffer
+    - make all_tests
+
+---
+kind: pipeline
+type: docker
+name: php-7.3
+
+clone:
+  disable: true
+
+steps:
+- name: test-php-7.3
+  image: nodiscc/drone-ci-base:7.3
+  commands:
+    - git clone https://github.com/nodiscc/Shaarli.git .
+    - git checkout $DRONE_COMMIT
+    - phpenv config-rm xdebug.ini || echo 'No xdebug config.'
+    - whoami
+    - id
+    - echo $PWD
+    - ls -ld $PWD
+    - ls -ld /drone
+    - echo $HOME
+    # install/update composer and php dependencies
+    - composer config --unset platform && composer config platform.php 7.3
+    - composer update
+    # - PATH=${PATH//:\.\/node_modules\/\.bin/}
+    # run tests
+    - make clean
+    - make check_permissions
+    - make code_sniffer
+    - make all_tests