Writing data to file in Dockerfile

It's because Docker load the entire context of the directory (where your Dockerfile is located) to Docker daemon at the beginning. From Docker docs,

The build is run by the Docker daemon, not by the CLI. The first thing a build process does is send the entire context (recursively) to the daemon. In most cases, it’s best to start with an empty directory as context and keep your Dockerfile in that directory. Add only the files needed for building the Dockerfile.

Since your text file was not available at the beginning, you get that error message. If you still want that text file want to be added to Docker image, you can call `docker build' command from the same script file. Modify script.sh,

#!/usr/bin/env bash
printf "blah 
blah 
blah 
blah\n" | sudo tee <docker-file-directory>/file.txt
docker build --tag yourtag <docker-file-directory>

And modify your Dockerfile just to add the generated text file.

ADD file.txt
.. <rest of the Dockerfile instructions>

When you RUN chmod 755 script.sh && ./script.sh it actually execute this script inside the docker container (ie: in the docker layer).

When you ADD file.txt . you are trying to add a file from your local filesystem inside the docker container (ie: in a new docker layer).

You can't do that because the file.txt doesn't exist on your computer.

In fact, you already have this file inside docker, try docker run --rm -ti mydockerimage cat file.txt and you should see it's content displayed