docker: Set has_appserver=0 correctly.

The docker installer configuration incorrectly had has_appserver set
to 0; this meant that (A) the docker-zulip code needed to copy the
block of code in the installer for the `has_appserver` case into the
Dockerfile (unnecessarily), and (B) one couldn't use `install` from a
Git ref (because the static asset compiler didn't end up in the right
place).

It appears that docker-zulip tried to set this flag in their `install`
command line, but the construction inside `install` meant that didn't
work.

scripts/lib/install

@@ -212,7 +212,7 @@ set -e
 if [ "$DEPLOYMENT_TYPE" = "dockervoyager" ]; then
     has_camo=1
     has_nginx=1
-    has_appserver=1
+    has_appserver=0
     has_rabbit=1
     has_postgres=1
 fi