Rebuilding an image with `dmake` results in a 2X storage overhead

[michaelsevilla]

After running:

dmake \
          -e GIT_URL="https://github.com/ceph/ceph.git" \
          -e RECONFIGURE="true" \
          -e SHA1_OR_REF="remotes/origin/master" \
          -e CONFIGURE_FLAGS="-DWITH_TESTS=OFF" \
          -e BUILD_THREADS=`grep processor /proc/cpuinfo | wc -l` \
          cephbuilder/ceph:latest \
          build-cmake

Then coding. Then trying to rebuild:

dmake cephbuilder/ceph:latest build-cmake

The images are twice as big:

[... snip ...] 96ce022cb184        About an hour ago   9.662 GB
[... snip ...] 8d7d34f3fea1        About an hour ago   5.433 GB