travis: build debian packages and documentation

The resulting packages are pushed to Debian repositories hosted on
bintray.com.

This is the first step to move away from our custom buildbot setup (see
https://i3wm.org/docs/buildbot.html for details on that) towards
infrastructure which is more standard (travis) and in the open.

travis/travis-base-ubuntu.Dockerfile

@@ -0,0 +1,30 @@
+# vim:ft=Dockerfile
+# Same as travis-base.Dockerfile, but without the test suite dependencies since
+# we only build Debian packages on Ubuntu, we don’t run the tests.
+FROM ubuntu:wily
+
+RUN echo force-unsafe-io > /etc/dpkg/dpkg.cfg.d/docker-apt-speedup
+# Paper over occasional network flakiness of some mirrors.
+RUN echo 'APT::Acquire::Retries "5";' > /etc/apt/apt.conf.d/80retry
+
+# NOTE: I tried exclusively using gce_debian_mirror.storage.googleapis.com
+# instead of httpredir.debian.org, but the results (Fetched 123 MB in 36s (3357
+# kB/s)) are not any better than httpredir.debian.org (Fetched 123 MB in 34s
+# (3608 kB/s)). Hence, let’s stick with httpredir.debian.org (default) for now.
+
+# Install mk-build-deps (for installing the i3 build dependencies),
+# clang and clang-format-3.5 (for checking formatting and building with clang),
+# lintian (for checking spelling errors),
+# test suite dependencies (for running tests)
+RUN apt-get update && \
+    DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+    dpkg-dev devscripts git equivs \
+    clang clang-format-3.5 \
+    lintian && \
+    rm -rf /var/lib/apt/lists/*
+
+# Install i3 build dependencies.
+COPY debian/control /usr/src/i3-debian-packaging/control
+RUN apt-get update && \
+    DEBIAN_FRONTEND=noninteractive mk-build-deps --install --remove --tool 'apt-get --no-install-recommends -y' /usr/src/i3-debian-packaging/control && \
+    rm -rf /var/lib/apt/lists/*