Updates Dockerfile to fix docker build, adds .dockerignore file and updates the readme with instructions for docker - #162

I realized I should caveat this PR slightly -- I am not a golang-native developer. You should review this code critically in that light, as though I am a junior developer who doesn't even know the ways in which this PR could be wrong.

It is very possible that I am missing something obvious about which folders should be hoisted from the code repository into the Dockerfile working directory prior to the the call to go mod download and go build -o scratchdata. For instance, I did not copy in the /models or /pkg folder when I was drafting this PR yesterday, because it appeared to work without them. However, reading though the import section of main.go this evening, I realize I probably should at least copy in the /pkg folder, otherwise a developer working on the code later might wonder why their changes in the /pkg/ folder are not having any effect.

Added .dockerignore file (same as the .gitignore file), and added a section in the readme on how to build and run a container using docker run.

Looks good! I'll give it a test and merge when I confirm. Appreciate your iterating on this :)