Answer by Daniel Katz for How to include files outside of Docker's build context?

In my case, my Dockerfile is written like a template containing placeholders which I'm replacing with real value using my configuration file.

So I couldn't specify this file directly but pipe it into the docker build like this:

sed "s/%email_address%/$EMAIL_ADDRESS/;" ./Dockerfile | docker build -t katzda/bookings:latest . -f -;

But because of the pipe, the COPY command didn't work. But the above way solves it by -f - (explicitly saying file not provided). Doing only - without the -f flag, the context AND the Dockerfile are not provided which is a caveat.