Allow developer to customize UID/GID for web app container

[eecavanna]

In this branch, I made it so the developer can customize which user PM2 is launched as within the web app container. I also included a commented-out sample snippet in the docker-compose.prod.yml file to facilitate the use of a spontaneously-built image instead of an image present in a registry.

[eecavanna]

Hi @mflynn-lanl, it is being done via the --ingroup CLI option to the adduser program.

# Create a group having the specified GID (Group ID) and group name, and create
# a user (in that group) having the specified UID (User ID) and user name.
# Reference: https://gist.github.com/utkuozdemir/3380c32dfee472d35b9c3e39bc72ff01
RUN addgroup -g $GROUP_ID $GROUP_NAME && \
    adduser --shell /sbin/nologin --disabled-password \
            --ingroup $GROUP_NAME --uid $USER_ID $USER_NAME

Here's the help snippet (source) for the adduser program:

adduser [OPTIONS] USER [GROUP]

Create new user, or add USER to GROUP

     -h --home DIR           Home directory
     -g --gecos GECOS        GECOS field
     -s --shell SHELL        Login shell named SHELL by example /bin/bash
     -G --ingroup GRP        Group (by name)
     -S --system             Create a system user
     -D --disabled-password  Don't assign a password, so cannot login
     -H --no-create-home     Don't create home directory
     -u --uid UID            User id
     -k SKEL                 Skeleton directory (/etc/skel)