I have a filed called ssh_key.txt and the content of that file is: my_private_key

In my Dockerfile I have this instruction:

RUN --mount=type=secret,id=ssh_key 
  cat /run/secrets/ssh_key > /home/${USER_NAME}/.ssh/git_key

When I build this image:

docker build --secret id=ssh_key,src=ssh_key.txt -t my_image.

And run the image, when I cd into the .ssh directory and run:

cat git_key

Weirdly the content is:

"��my_private_key"

I don’t understand where the �� is coming from, does anyone know I could be doing wrong?