Dockerfile COPY instruction failing?

I couldn't get COPY to work until I understood the context (I was trying to copy a file from outside of the context)

The docker build command builds an image from a Dockerfile and a context. The build’s context is the files at a specified location PATH. The PATH is a directory on your local filesystem.

A context is processed recursively. So, a PATH includes any subdirectories.

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.

Warning: Do not use your root directory, /, as the PATH as it causes the build to transfer the entire contents of your hard drive to the Docker daemon.

Reference: https://docs.docker.com/engine/reference/builder/#usage

At the time I originally wrote this, Docker didn’t expand ~ or $HOME. Now it does some expansions inside the build context, but even so they are probably not what you want—they aren’t your home directory outside the context. You need to reference the file explicitly, or package it relative to the Dockerfile itself.

Docker can only copy files from the context, the folder you are minus any file listed in the dockerignore file.

When you run 'docker build' docker tars the context and it sends it to the docker daemon you are connected to. It only lets you copy files inside of the context because the daemon might be a remote machine.