dockerfile: bump to Go 1.12

jzelinskie · jzelinskie · commit 4a975535f17c · 2019-05-14T14:57:03.000-04:00
diff --git a/Dockerfile b/Dockerfile
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM golang:1.7-alpine
+FROM golang:1.12-alpine
 
 MAINTAINER Quentin Machu <quentin.machu@coreos.com>
 
