README steps depend on `docker login`

[ivanistheone]

Observed:

I'm trying this now:

docker build -t  softcover-docker .
docker run -v `pwd`:/book -d -p 4000:4000 softcover-docker sc server

if it works I will PR it...

[ivanistheone]

Okay the problem was use of tag name softcover instead of full name softcover/softcover:

This is the commit that fixes instructions in README:
771dec8