Update the TravisCI environment to use Docker

The Travis environment is terribly outdated, and they are not really
subtly pushing people to use Docker to set up their own containerised CI
environments instead of pulling from Ubuntu 12.04 or 14.04.

Let's try using this approach:

  - create a Docker image that pulls from Debian Stretch
  - set up a build and test environment
  - push the image to the Docker Hub
  - create a derived Docker image that copies the Epoxy repo
    when running under Travis
  - run the build and test script inside the derived image

This is similar to what Meson does for its CI.

.travis.yml

@@ -1,28 +1,22 @@
-language: c
+sudo: false
+
+os:
+  - linux
+
 compiler:
   - gcc
-  - clang
-
-sudo: false
 
-addons:
-  apt:
-    packages:
-      - xutils-dev
-      - libgl1-mesa-dev
-      - libegl1-mesa-dev
-      - libgles1-mesa-dev
-      - libgles2-mesa-dev
-      - libgl1-mesa-dri
+language:
+  - c
 
-env:
-  global:
-    - DISPLAY=:99.0
+services:
+  - docker
 
 before_install:
-  - "/sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -ac -screen 0 1280x1024x24"
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker pull ebassi/epoxyci ; fi
 
 script:
-  - NOCONFIGURE=1 ./autogen.sh
-  - mkdir _build && cd _build
-  - ../configure && make V=1 && make V=1 check
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then echo FROM ebassi/epoxyci > Dockerfile ; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then echo ADD . /root >> Dockerfile ; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker build -t withgit . ; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker run withgit /bin/sh -c "cd /root && TRAVIS=true CC=$CC CXX=$CXX ./epoxy-run-tests.sh" ; fi