[readme] Fix a Docker warning from using Docker installation instructions

Fix a LegacyKeyValueFormat warning when building a Docker image using the 'Installing In Docker' instructions.

The warning fixed is: `LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format`

See: [LegacyKeyValueFormat](https://docs.docker.com/reference/build-checks/legacy-key-value-format/)

Co-authored-by: clifennel <li.chr+github@northeastern.edu>
Co-authored-by: Jordan Harband <ljharb@gmail.com>

Author: clifennel

Dockerfile

@@ -15,14 +15,14 @@ LABEL version="latest"
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 
 # Prevent dialog during apt install
-ENV DEBIAN_FRONTEND noninteractive
+ENV DEBIAN_FRONTEND=noninteractive
 
 # ShellCheck version
 ENV SHELLCHECK_VERSION=0.7.0
 
 # Pick a Ubuntu apt mirror site for better speed
 # ref: https://launchpad.net/ubuntu/+archivemirrors
-ENV UBUNTU_APT_SITE ubuntu.cs.utah.edu
+ENV UBUNTU_APT_SITE=ubuntu.cs.utah.edu
 
 # Replace origin apt package site with the mirror site
 RUN sed -E -i "s/([a-z]+.)?archive.ubuntu.com/$UBUNTU_APT_SITE/g" /etc/apt/sources.list
@@ -92,7 +92,7 @@ RUN echo 'nvm ALL=(ALL) NOPASSWD: ALL' >> /etc/sudoers
 USER nvm
 
 # Create a script file sourced by both interactive and non-interactive bash shells
-ENV BASH_ENV /home/nvm/.bash_env
+ENV BASH_ENV=/home/nvm/.bash_env
 RUN touch "$BASH_ENV"
 RUN echo '. "$BASH_ENV"' >> "$HOME/.bashrc"
 

README.md

@@ -148,7 +148,7 @@ When invoking bash as a non-interactive shell, like in a [Docker](https://www.do
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 
 # Create a script file sourced by both interactive and non-interactive bash shells
-ENV BASH_ENV "${HOME}/.bash_env"
+ENV BASH_ENV="${HOME:-/root}/.bash_env"
 RUN touch "${BASH_ENV}"
 RUN echo '. "${BASH_ENV}"' >> ~/.bashrc
 
@@ -174,7 +174,7 @@ RUN apt update && apt install curl -y
 RUN curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.40.8/install.sh | bash
 
 # set env
-ENV NVM_DIR=/root/.nvm
+ENV NVM_DIR="${HOME:-/root}/.nvm"
 
 # install node
 RUN bash -c "source $NVM_DIR/nvm.sh && nvm install $NODE_VERSION"