Improve add-auth usage message

#!/bin/sh

set -eu

die() { echo "$USAGE"; echo "Fatal: $*"; exit 1; }

trap 'test $? -eq 2 && echo >&2 "$USAGE"' EXIT

USAGE="$0 REPOSITORY USERNAME

USAGE="

Usage: $0 REPOSITORY USERNAME

REPOSITORY  The image repository to authenticate against

USERNAME    The username to authenticate with

The password to authenticate with will be read from STDIN

"

CONFIG=$DOCKER_CONFIG/config.json || die "DOCKER_CONFIG must be set in the environment"

REPOSITORY=$1 || die "REPOSITORY is missing"

USERNAME=$2 || die "USERNAME is missing"

CONFIG=$DOCKER_CONFIG/config.json

REPOSITORY=${1?"REPOSITORY is missing"}

USERNAME=${2?"USERNAME is missing"}

read -p "Enter password: " PASSWORD