Document decision to mutate docker container

[Erikvv]

In response to #3 write down why we ship a docker image which downloads binaries instead of having them in the image. In the links at storj/storj#4489 there should be enough information to put it together.

Put this information in the README of this repo and possibly other places.

[profclems]

We already have a hard decision document on confluence: https://storjlabs.atlassian.net/wiki/spaces/PM/pages/2269151269/Storage+node+rollout+support+for+existing+Docker+nodes.
We can either make it public and point to it in the README, or move it to this repo in DESIGN_DECISION.md and point to it in the README

[Erikvv]

I would reformat it a little, putting the option we chose at the top. But sure most of the content can be put here.