update c/buildah to v1.12.0

Also bump docker/docker.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

vendor/github.com/containers/buildah/changelog.txt

@@ -1,3 +1,29 @@
+- Changelog for v1.12.0 (2019-12-13)
+  * Allow ADD to use http src
+  * Bump to c/storage v.1.15.3
+  * install.md: update golang dependency
+  * imgtype: reset storage opts if driver overridden
+  * Start using containers/common
+  * overlay.bats typo: fuse-overlays should be fuse-overlayfs
+  * chroot: Unmount with MNT_DETACH instead of UnmountMountpoints()
+  * bind: don't complain about missing mountpoints
+  * imgtype: check earlier for expected manifest type
+  * Vendor containers/storage fix
+  * Vendor containers/storage v1.15.1
+  * Add history names support
+  * PR takeover of #1966
+  * Tests: Add inspect test check steps
+  * Tests: Add container name and id check in containers test steps
+  * Test: Get permission in add test
+  * Tests: Add a test for tag by id
+  * Tests: Add test cases for push test
+  * Tests: Add image digest test
+  * Tests: Add some buildah from tests
+  * Tests: Add two commit test
+  * Tests: Add buildah bud with --quiet test
+  * Tests: Add two test for buildah add
+  * Bump back to v1.12.0-dev
+
 - Changelog for v1.11.6 (2019-12-03)
   * Handle missing equal sign in --from and --chown flags for COPY/ADD
   * bud COPY does not download URL