Use secret seed for internal database passwords

Getting root-only read permissions into the Postgres image would
be tricky due to its init script nature. But it isn't exposed to
the outside anyway.
diff --git a/postgres/docker/entrypoint b/postgres/docker/entrypoint
index 1782d31..be591bc 100644
--- a/postgres/docker/entrypoint
+++ b/postgres/docker/entrypoint
@@ -2,16 +2,7 @@
 
 set -e
 
+# This master password won't be known to anyone
 export POSTGRES_PASSWORD=$(mktemp -u XXXXXXXXXXXXXXXX)
 
-kc_secret=/tmp/passwd/keycloak/secret
-{
-    if [ ! -f ${kc_secret} ]; then
-        mkdir -p $(dirname ${kc_secret})
-        echo $(mktemp -u XXXXXXXXXXXXXXXX) >${kc_secret}
-    fi
-    chown root:postgres ${kc_secret}
-    chmod 444 ${kc_secret}
-}
-
 exec docker-entrypoint.sh "$@"