Since docker is run from the top-level of the otp repository, the
entire repository will be the context for the docker daemon. In my
private repository, more than 800Mb had to be copied before the
build could be started.

Let the ./scripts directory be the context for docker instead.
That way, only the otp.tar.gz file needs to be copied.

While we are it, also use the ADD command in the Dockerfiles to
pack up the tar file and avoid copying the tar file itself to
the Docker image.

Pass all argument except the first as the command for "docker run".

Running fewer jobs in parallel reduces the risk of running out
of memory.

Travis CI will kill build jobs that have not produced output for
10 minutes.

The OTP build was never killed because of inactivity before 74796de9c7739
(which started to capture the output in a temporary file). After that
commit, now and then a build would be killed because it did not finish
in 10 minutes.

Update the build script to periodically print a ".", but only if the
size of the log file has changed. That way, if there is a real hanging
during the build, Travis CI will still kill the build.

Alternatives considered: Using travis_wait. Rejected because travis_wait
will extend the build unconditonally, even if the build does not progress
at all.

The huge log files are problematic. Add shell scripts that
captures the log output to a temporary file and only display
the log file if an error occurs.