Add blog posts about Binder federation member and docker stacks

[choldgraf]

This provides two draft blog posts to describe upstream impact that 2i2c has been involved with.

1. A historical post about our support for Docker Stacks (so that we have a record of this happening at all)
2. A short post describing the Hetzner experiment for a mybinder.org federation member

If either of these is controversial we can split this into 2 PRs but I thought it'd be easy-enough to add a quick stub post about Docker Stacks first.

[choldgraf]

I've updated the post to focus more on the single-node workflow rather than one specific cloud provider. I think the post should be ready to go whenever @yuvipanda thinks it's OK.