As suggested in
https://gitlab.com/BuildStream/buildstream/merge_requests/857#note_106809743,
add a NEWS entry for `contrib/bst-docker-import` that was added
recently.

Add a new section under "Additional writings" section about how
BuildStream integrates with Docker. At present, this section includes
only two sections:

- "Run BuildStream inside Docker", that refers to existing instructions
for doing so
- "Generate Docker images", that adds instructions for using the newly
added `contrib/bst-docker-import` script

In future, this document can be extended to include the following
things:

- Importing Docker sources via `docker` source plugin - if/when the
Docker source plguin is moved from bst-external to BuildStream proper
- A Docker sandbox, if/when that is added

element.py: Prepare local sandbox for bst checkout and bst shell

See merge request BuildStream/buildstream!860

A remote execution sandbox is not suitable for these commands.

Add contrib script to generate Docker images from bst checkout

See merge request BuildStream/buildstream!857

This script can be useful to generate a Docker image from `bst checkout`
of an element as a single command. While this script does not eliminate
the need for a proper Docker/OCI element plugin that would probably also
support layering, it provides a cheap way to export the element into a
Docker image.

Catch correct exception from shutil.rmtree

Closes #153

See merge request BuildStream/buildstream!849

Python documentation is not clear on what shutil.rmtree is supposed to
raise. However from the source code, it is obvious that OSError are
raised, but never shutil.Error.

It is not easy to test in normal condition because issues happen
usually in combination with a FUSE filesystem, a probably a race
condition where `fstatat` returns an error just before the filesystem
being unmounted.

Fixes #153.

Do not save workspace on failed build

Closes #634

See merge request BuildStream/buildstream!812