Avoid COPY in apt-pining as suggested by @tianon

see https://github.com/docker-library/official-images/pull/444 for details

library/r-base

@@ -1,6 +1,6 @@
 # maintainer: "Carl Boettiger" <[email protected]> (@cboettig)
 # maintainer: "Dirk Eddelbuettel" <[email protected]> (@eddelbuettel)
 
-latest: git://github.com/rocker-org/rocker@56cb59cc262e9d2c02f9e11cfacab65f0b659c5f r-base
-3.1.2: git://github.com/rocker-org/rocker@56cb59cc262e9d2c02f9e11cfacab65f0b659c5f r-base
+latest: git://github.com/rocker-org/rocker@fa244c6a6010d41fca568d2469be681959a1f3c4 r-base
+3.1.2: git://github.com/rocker-org/rocker@fa244c6a6010d41fca568d2469be681959a1f3c4 r-base