travis: check spelling of binaries and manpages, use docker

We now build a docker base container based on debian sid (where the very
latest packages are available). That base container is updated once a
month, or whenever travis-build.Dockerfile or debian/control change, but
re-used for subsequent travis runs. While the initial build might take
up to 15 minutes, subsequent builds typically run in a minute or two.

All the different steps that we run on travis are now factored into
separate scripts in the travis/ directory.

Switching to docker should also help with issue #2174.

travis/docker-build-and-push.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+set -e
+
+# .dockerignore is created on demand so that release.sh and other scripts are
+# not influenced by our travis setup.
+echo .git > .dockerignore
+
+docker build --pull --no-cache --rm -t=${BASENAME} -f travis-build.Dockerfile .
+docker login -e ${DOCKER_EMAIL} -u ${DOCKER_USER} -p ${DOCKER_PASS}
+docker push ${BASENAME}