[readme] use tilde expansion instead of "$HOME" for consistency

at least in this code block.

README.md

@@ -146,7 +146,7 @@ When invoking bash as a non-interactive shell, like in a Docker container, none
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 
 # Create a script file sourced by both interactive and non-interactive bash shells
-ENV BASH_ENV $HOME/.bash_env
+ENV BASH_ENV ~/.bash_env
 RUN touch "${BASH_ENV}"
 RUN echo '. "${BASH_ENV}"' >> ~/.bashrc