docker-entrypoint.sh

#!/usr/bin/env bash
set -e

debug=${DEBUG_MODE-false}

[[ $debug = 'true' ]] && echo "[DEBUG] Container running in DEBUG_MODE"

# this if will check if the first argument is a flag
# but only works if all arguments require a hyphenated flag
# -v; -SL; -f arg; etc will work, but not arg1 arg2
if [ "${1:0:1}" = '-' ]; then
    set -- ssh-agent "$@"
fi

# check if running ssh-agent and run as appropriate user and group
if [ "$1" = 'ssh-agent' ]; then

  # Podman handles user and group ID mapping from the host, so we don't need to do any of the below if using it.
  if [ "$container" = 'podman' ]; then
    exec "$@"
  fi

  # Check username passthrough
  if [ -z "${HOST_USER_NAME}" ]; then
    echo '$HOST_USER_NAME is not set, defaulting to "user"'
    HOST_USER_NAME=user
  elif [[ $HOST_USER_NAME =~ ^[0-9]+$ ]]; then
    # If only digits, use default
    HOST_USER_NAME=user
  else
    # Clean user name
    CLEAN=${HOST_USER_NAME//_/}
    # Replace spaces with underscores
    CLEAN=${CLEAN// /_}
    # Clean out anything that's not alphanumeric or an underscore
    CLEAN=${CLEAN//[^a-zA-Z0-9_]/}
    # Lowercase with TR
    CLEAN=`echo -n $CLEAN | tr A-Z a-z`
    HOST_USER_NAME=$CLEAN
  fi

  # Check groupname passthrough
  if [ -z "${HOST_GROUP_NAME}" ]; then
    echo '$HOST_GROUP_NAME is not set, defaulting to "group"'
    HOST_GROUP_NAME=group
  elif [[ $HOST_GROUP_NAME =~ ^[0-9]+$ ]]; then
    # If only digits, use default
    HOST_GROUP_NAME=group
  else
    # Clean group name
    CLEAN=${HOST_GROUP_NAME//_/}
    # Replace spaces with underscores
    CLEAN=${CLEAN// /_}
    # Clean out anything that's not alphanumeric or an underscore
    CLEAN=${CLEAN//[^a-zA-Z0-9_]/}
    # Lowercase with TR
    CLEAN=`echo -n $CLEAN | tr A-Z a-z`
    HOST_GROUP_NAME=$CLEAN
  fi

  # Check home directory passthrough
  if [ -z "${HOST_HOME_DIRECTORY}" ]; then
    echo '$HOST_HOME_DIRECTORY is not set, defaulting to "/home/$HOST_USER_NAME"'
    HOST_HOME_DIRECTORY="/home/$HOST_USER_NAME"
  fi

  # Use an internal group if one matches, or create one with the correct GID if not
  if [ -z "${HOST_GROUP_ID}" ]; then
    echo '$HOST_GROUP_ID not set, unable to map external and internal GIDs'
    echo 'Using default GID of 1000 instead'
    HOST_GROUP_ID=1000
    [[ $debug = 'true' ]] && echo "[DEBUG] groupadd -g $HOST_GROUP_ID $HOST_GROUP_NAME"
    groupadd -g $HOST_GROUP_ID $HOST_GROUP_NAME
  else
    if getent group $HOST_GROUP_ID >/dev/null 2>&1; then
      HOST_GROUP_NAME=$(getent group $HOST_GROUP_ID | cut -d: -f1)
      echo "Matching internal group found, running as $HOST_GROUP_NAME"
    else
      echo "No matching internal group found, creating one..."
      [[ $debug = 'true' ]] && echo "[DEBUG] groupadd -g $HOST_GROUP_ID $HOST_GROUP_NAME"
      groupadd -g $HOST_GROUP_ID $HOST_GROUP_NAME
    fi
  fi

  # Use an internal user if one matches, or create one with the correct UID and GID if not
  if [ -z "${HOST_USER_ID}" ]; then
    echo '$HOST_USER_ID not set, unable to map external and internal UIDs'
    echo 'Using default UID of 1000 instead'
    HOST_USER_ID=1000
    [[ $debug = 'true' ]] && echo "[DEBUG] useradd -l -m -s /bin/bash -u $HOST_USER_ID -g $HOST_GROUP_ID -d $HOST_HOME_DIRECTORY $HOST_USER_NAME"
    useradd -l -m -s /bin/bash -u $HOST_USER_ID -g $HOST_GROUP_ID -d $HOST_HOME_DIRECTORY $HOST_USER_NAME
  else
    # Use an existing internal user if one matches
    if id -u $HOST_USER_ID >/dev/null 2>&1; then
      HOST_USER_NAME=$(id -un $HOST_USER_ID)
      echo "Matching internal user found, running as $HOST_USER_NAME"
    else
      echo "No matching interneral user found, creating one..."
      [[ $debug = 'true' ]] && echo "[DEBUG] useradd -l -m -s /bin/bash -u $HOST_USER_ID -g $HOST_GROUP_ID -d $HOST_HOME_DIRECTORY $HOST_USER_NAME"
      useradd -l -m -s /bin/bash -u $HOST_USER_ID -g $HOST_GROUP_ID -d $HOST_HOME_DIRECTORY $HOST_USER_NAME
    fi
  fi

  echo "Running ssh-agent as $HOST_USER_NAME:$HOST_GROUP_NAME"
  exec su-exec $HOST_USER_NAME:$HOST_GROUP_NAME "$@"
fi

# else default to run whatever the user wanted like "bash"
exec "$@"