Consider using the default GitHub Docker image for dev containers to reduce storage costs

[StevenMaude]

You don't incur a storage cost for using this image.

Using such an image may reduce the cost of:

• running and stopped Codespaces
• prebuilds (#21) should we use them

What's not clear is whether extending the image incur the cost of the whole combined image, or just the extra layers?

It may be that this image is entirely unsuitable. But it's worth at least considering if we're starting to review and improve our setup.

[StevenMaude]

Closing as a duplicate of #33 which has more detail.