Update Docker build script to use fakelogin

fakelogin populates /var/run/utmp enough such that getlogin() works, and
is included in the latest Docker image.

Run `docker pull andschwa/magrathea` to update.

scripts/build.sh

@@ -1,4 +1,9 @@
 #!/usr/bin/env sh
 
 # Docker requires the volume path to be absolute... so we resolve it ourselves.
-docker run --rm --interactive --tty --volume $(dirname $(pwd))/:/opt/monad --workdir /opt/monad/scripts andschwa/magrathea:latest $@
+docker run --rm --interactive --tty \
+       --volume $(dirname $(pwd))/:/opt/monad \
+       --workdir /opt/monad/scripts \
+       andschwa/magrathea:latest \
+       bash -c "/opt/fakelogin; $*"
+# we use $* over $@ so that multi-word parameters aren't split up