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This option allows us to specify a custom Dockerfile when building the image.

Just reformat it across multiple lines. This makes it easier to read.

When we call sys.exit(), we raise a SystemExit exception. The result
was that we entered the except-block, which also reported a failure to
Zabbix.

We aren't going to build the images so often, so the cache doesn't help
us very much. On the other hand, it leads us to getting errors
because it uses a cache of a very old Debian package list.

Hostname support

See merge request !1

Docker actually supports passing environment variables without a value
to make it load them from the environment. Use that functionality.

Python defaults to buffered stdout when the output isn't a terminal. This
causes the output from python to come after the processes it has executed.
Change it to unbuffered, so that the output is printed immediately.

By not deleting the container immediately, we avoid a Docker bug
that causes «docker run --rm» to fail occasionally.

Instead, we attempt to delete the container on the next run. We also
don't fail if the container deletion fails. Instead, we hope that it
will be deleted at a later run.

These parameters were supposed to be optional, so ensure that the
default value is the empty list.