Major update to stretch and many improvements

Improvements:
* Update to debian stretch
* Fix Dockerfile style
* Add FTP UID/GID Docker build args
* Add run-ssl to Makefile for testing SSL config
* Fix scripts style
* Fix scripts issues reported by shellcheck
* Add FTP_CHOWN_ROOT function to chown /srv in the container
* Add allow_writeable_chroot=YES to vsftpd_ssl.conf to match vsftpd.conf
* Update README.md

add-virtual-user.sh

@@ -1,23 +1,24 @@
 #!/usr/bin/env bash
+# Adds a virtual ftp user to /etc/vsftpd/virtual-users.db
 
 set -e
 
-[ "$DEBUG" == 'true' ] && set -x
+[[ "${DEBUG}" == "true" ]] && set -x
 
-DB=/etc/vsftpd/virtual-users.db
+DB="/etc/vsftpd/virtual-users.db"
 
-if [ "$#" -lt 2 ] || [ "$#" -gt 3 ]; then
-    echo "Usage: $0 [-d] <user> <password>" >&2
-    echo >&2
-    echo "[ -d ] Delete the database first" >&2
-    exit 1
+if [[ "${#}" -lt 2 ]] || [[ "${#}" -gt 3 ]]; then
+  echo "Usage: $0 [-d] <user> <password>" >&2
+  echo >&2
+  echo "[ -d ] Delete the database first" >&2
+  exit 1
 fi
 
-if [ "$1" == "-d" ]; then
-    if [ -f $DB ]; then
-        rm $DB
-    fi
-    shift
+if [[ "${1}" == "-d" ]]; then
+  if [[ -f "${DB}" ]]; then
+    rm "${DB}"
+  fi
+  shift
 fi
 
-echo -e "$1\n$2" | db5.3_load -T -t hash $DB
+printf '%s\n%s\n' "${1}" "${2}" | db5.3_load -T -t hash "${DB}"