doc: Add new document about Docker integrations

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