Fix default site alts in Dockerfile

The Dockerfile was incorrectly assigning an empty string to the site alt
values, which caused the filter to update relevant sites with an empty
host. This replaces the empty string with the correct domain for each
site alternative.

Dockerfile

@@ -39,11 +39,11 @@ ENV HTTPS_ONLY=$use_https
 ARG whoogle_port=5000
 ENV EXPOSE_PORT=$whoogle_port
 
-ARG twitter_alt=''
+ARG twitter_alt='nitter.net'
 ENV WHOOGLE_ALT_TW=$twitter_alt
-ARG youtube_alt=''
+ARG youtube_alt='invidious.snopyta.org'
 ENV WHOOGLE_ALT_YT=$youtube_alt
-ARG instagram_alt=''
+ARG instagram_alt='bibliogram.art/u'
 ENV WHOOGLE_ALT_YT=$instagram_alt
 
 COPY . .