Cirrus: Use branch-specific container tags

Automated building of container images is handled in quay.io based on
changes in the master branch of this repository.  However, as additional
branches are made, the "latest" image (from master) diverges from their
expectations.  Fix this by using the branch-tagged images built by quay.
For the near-term, this also implies quay.io will be configured to also
build different images for each branch, and tag them appropriately.
Long-term, image build automation should be combined with libpod
automation - to avoid needing to maintain automation in multiple
systems/locations.

Signed-off-by: Chris Evich <cevich@redhat.com>

.cirrus.yml

@@ -92,7 +92,7 @@ gating_task:
 
     # Runs within Cirrus's "community cluster"
     container:
-        image: "quay.io/libpod/gate:latest"
+        image: "quay.io/libpod/gate:master"
         cpu: 4
         memory: 12
 
@@ -184,7 +184,7 @@ varlink_api_task:
 
     # Runs within Cirrus's "community cluster"
     container:
-        image: "quay.io/libpod/gate:latest"
+        image: "quay.io/libpod/gate:master"
         cpu: 4
         memory: 12
 
@@ -273,7 +273,7 @@ meta_task:
         - "build_without_cgo"
 
     container:
-        image: "quay.io/libpod/imgts:latest"  # see contrib/imgts
+        image: "quay.io/libpod/imgts:master"  # see contrib/imgts
         cpu: 1
         memory: 1
 
@@ -309,7 +309,7 @@ image_prune_task:
         - "meta"
 
     container:
-        image: "quay.io/libpod/imgprune:latest"  # see contrib/imgprune
+        image: "quay.io/libpod/imgprune:master"  # see contrib/imgprune
         cpu: 1
         memory: 1
 
@@ -714,7 +714,7 @@ success_task:
         GOSRC: "/go/src/github.com/containers/libpod"
 
     container:
-        image: "quay.io/libpod/gate:latest"
+        image: "quay.io/libpod/gate:master"
         cpu: 1
         memory: 1