[readme] do not use tilde expansion in ENV of Dockerfile

It won't expand. my bad.

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 ~/.bash_env
+ENV BASH_ENV "${HOME}/.bash_env"
 RUN touch "${BASH_ENV}"
 RUN echo '. "${BASH_ENV}"' >> ~/.bashrc