Update default bibliogram link in Dockerfile

Bibliogram uses a slightly different URL format than Instagram, and
requires a "u/" before the username when replacing Instagram links. This
was already implemented everywhere else except the Dockerfile.

Dockerfile

@@ -37,7 +37,7 @@ ARG use_https=''
 ARG whoogle_port=5000
 ARG twitter_alt='farside.link/nitter'
 ARG youtube_alt='farside.link/invidious'
-ARG instagram_alt='farside.link/bibliogram'
+ARG instagram_alt='farside.link/bibliogram/u'
 ARG reddit_alt='farside.link/libreddit'
 ARG medium_alt='farside.link/scribe'
 ARG translate_alt='farside.link/lingva'