docker: add alpine,debian,ubuntu test images

Relates to https://github.com/shaarli/Shaarli/issues/843

Added:
- Makefile target to run commands in a Docker test context
- Docker images to run Shaarli test suites:
  - Alpine 3.6
  - Debian 8
  - Debian 9
  - Ubuntu 16.04
- Documentation

Signed-off-by: VirtualTam <virtualtam@flibidi.net>

doc/md/Unit-tests-Docker.md

@@ -0,0 +1,56 @@
+## Running tests inside Docker containers
+
+Read first:
+
+- [Docker 101](docker/docker-101.md)
+- [Docker resources](docker/resources.md)
+- [Unit tests](Unit-tests.md)
+
+### Docker test images
+
+Test Dockerfiles are located under `docker/tests/<distribution>/Dockerfile`,
+and can be used to build Docker images to run Shaarli test suites under common
+Linux environments.
+
+Dockerfiles are provided for the following environments:
+
+- `alpine36` - [Alpine 3.6](https://www.alpinelinux.org/downloads/)
+- `debian8` - [Debian 8 Jessie](https://www.debian.org/DebianJessie) (oldstable)
+- `debian9` - [Debian 9 Stretch](https://wiki.debian.org/DebianStretch) (stable)
+- `ubuntu16` - [Ubuntu 16.04 Xenial Xerus](http://releases.ubuntu.com/16.04/) (LTS)
+
+What's behind the curtains:
+
+- each image provides:
+    - a base Linux OS
+    - Shaarli PHP dependencies (OS packages)
+    - test PHP dependencies (OS packages)
+    - Composer
+- the local workspace is mapped to the container's `/shaarli/` directory,
+- the files are rsync'd to so tests are run using a standard Linux user account
+  (running tests as `root` would bypass permission checks and may hide issues)
+- the tests are run inside the container.
+
+### Building test images
+
+```bash
+# build the Debian 9 Docker image
+$ cd /path/to/shaarli
+$ cd docker/test/debian9
+$ docker build -t shaarli-test:debian9 .
+```
+
+### Running tests
+
+```bash
+$ cd /path/to/shaarli
+
+# install/update 3rd-party test dependencies
+$ composer install --prefer-dist
+
+# run tests using the freshly built image
+$ docker run -v $PWD:/shaarli shaarli-test:debian9 docker_test
+
+# run the full test campaign
+$ docker run -v $PWD:/shaarli shaarli-test:debian9 docker_all_tests
+```