Add docker support

- Allow setup script to get credentials from flags
- Add Dockerfile and docker_entrypoint
- Edit README for forked version

scripts/register_feed2toot_app

@@ -34,6 +34,9 @@ parser.add_argument('--version', action='version', version=__version__)
 parser.add_argument('--client-credentials-file', dest='clientcredfile', help='the name of the client credentials for the Mastodon app', default='feed2toot_clientcred.txt')
 parser.add_argument('--user-credentials-file', dest='usercredfile', help='the name of the user credentials for the Mastodon app', default='feed2toot_usercred.txt')
 parser.add_argument('--name', help='the name of the Mastodon app', default='feed2toot')
+parser.add_argument('--instance', help='the URL of the Mastodon instance')
+parser.add_argument('--username', help='the username of your Mastodon account')
+parser.add_argument('--password', help='the password of your Mastodon account')
 opts = parser.parse_args()
 
 clientcredfile=opts.clientcredfile
@@ -44,7 +47,9 @@ print(headline)
 
 
 # get the instance
-instance = input('Mastodon instance URL (defaults to https://mastodon.social): ')
+instance = opts.instance
+if not instance:
+    instance = input('Mastodon instance URL (defaults to https://mastodon.social): ')
 if not instance:
     instance = 'https://mastodon.social'
 elif not instance.startswith('http'):
@@ -52,19 +57,25 @@ elif not instance.startswith('http'):
 
 # get the username
 userok = False
+quit_on_error = True
 while not userok:
-    user = input('Mastodon login: ')
+    user = opts.username
+    if not user:
+        user = input('Mastodon login: ')
+        quit_on_error = False
     if not user:
         print('Your Mastodon username can not be empty.')
-        userok = False
     elif '@' not in user or '.' not in user:
         print('Your Mastodon username should be an email.')
-        userok = False
     else:
         userok = True
+    if not userok and quit_on_error:
+        exit()
 
 # get the password
-password = getpass(prompt='Mastodon password: ')
+password = opts.password
+if not password:
+    password = getpass(prompt='Mastodon password: ')
 Mastodon.create_app(
       opts.name,
       api_base_url=instance,