Should build official Docker image and add usage instructions

[hoatle]

I created this: https://github.com/teracyhq/docker-files/tree/master/httpie-jwt-auth

and I'd like to do the same for official Docker image of httpie (instead of under teracy/ umbrella)

Let's discuss here for anyone interested and I could lead the effort.

related: https://github.com/jakubroztocil/httpie/pull/236

[hoatle]

Otherwise, let me maintain teracy/httpie Docker image as outside of the upstream repo and will link back to this project docs, I guess it's preferrable (as discussed from #236)

We use alpine so that the Docker image is very small (about 30MB in size): https://hub.docker.com/r/teracy/httpie-jwt-auth/tags/

[hoatle]

we need to make sure that the http alias to docker container should work the same as http native Python. If not, we should not introduce Docker image usage (or mention the limitations)