Embellish introduction

[bdevans]

Just a few small changes to the introduction.

A couple of points we might consider addressing in the introduction:

1. Why containers and not VMs e.g. easily version controllable and to an extent self-documenting, less bulk / minimal information to reproduce the environment etc.
2. The rise of containers' popularity and therefore ease of integration in other parts of the ecosystem e.g. their use in CI, development in VS Code, mybinder.org etc.

[nuest]

@bdevans Re. your "other points": I agree these might be interesting for readers to learn about, but we should weigh additional context against what we already have (are these points more important than an existing argument) and if we can add them in a succinct way. If you want to make a stab, that would be great. If you have time though, adding a couple of examples would also be helpful.