Supply pointer to APT instructions, fix minor doc nits.

README.md

@@ -13,10 +13,14 @@ whether in a new or existing installation, so all we really need to do
 is arrange to defer running the postinst script until the container
 starts up.
 
-Type `make` to build the image.  If you're paranoid, you might want to
-generate your own `debian:stretch` base image using `debootstrap`
-rather than trusting the one that's available on Dockerhub, but that's
-your call.
+If you're paranoid, you might want to generate your own
+`debian:stretch` base image first, using `debootstrap` rather than
+trusting the one that's available on Dockerhub, but that's your call.
+
+The `Makefile` assumes that you've configured APT according to the
+instructions at <https://download.rpki.net/APTng/debian/> so that
+`apt-get download` can find the package.  Type `make` to build the
+image.
 
 See `create.sh` for an example of how one might start up the generated
 container.  One of the things that would probably horrify a True

startup.sh

@@ -1,13 +1,11 @@
 #!/bin/sh -
 #
 # Startup script for rpki-rp running under Docker.
-#
-# This assumes that cron is already running, and that this script is running as root.
-# Most likely this script is running under cron as a @reboot action.
 
 # Start non-RPKI daemons.  postgresql in particular needs to be up
-# before the RPKI code so that we can check the database and configure
-# it if necessary.
+# before the running the rpki-rp postinst script, so that postinst can
+# look for the database, configure it if necessary, perform database
+# migrations, et cetera.
 
 for i in rsyslog postgresql xinetd apache2
 do
@@ -20,6 +18,6 @@ done
 
 dpkg --configure --pending
 
-# The rest of rpki-rp runs under cron
+# The rest of rpki-rp runs under cron.
 
 exec /usr/sbin/cron -f -L 15