[MXNET-849] - Enable armv7 with pinned docker images. (#12272)

* Revert "Temporarily disable ARMv7 builds (#12260)"

This reverts commit a4aced7.

* [MXNET-849] Pin armv7 dockcross image

Co-authored-by: Anton Chernov <mechernov@gmail.com>

Jenkinsfile

@@ -363,16 +363,16 @@ core_logic: {
         }
       }
     },
-    // 'ARMv7':{
-    //   node(NODE_LINUX_CPU) {
-    //     ws('workspace/build-ARMv7') {
-    //       timeout(time: max_time, unit: 'MINUTES') {
-    //         utils.init_git()
-    //         utils.docker_run('armv7', 'build_armv7', false)
-    //       }
-    //     }
-    //   }
-    // },
+    'ARMv7':{
+      node(NODE_LINUX_CPU) {
+        ws('workspace/build-ARMv7') {
+          timeout(time: max_time, unit: 'MINUTES') {
+            utils.init_git()
+            utils.docker_run('armv7', 'build_armv7', false)
+          }
+        }
+      }
+    },
     'ARMv6':{
       node(NODE_LINUX_CPU) {
         ws('workspace/build-ARMv6') {

ci/docker/Dockerfile.build.armv7

@@ -18,7 +18,7 @@
 #
 # Dockerfile to build MXNet for Android ARMv7
 
-FROM dockcross/linux-armv7
+FROM mxnetci/dockcross-linux-armv7:08212018
 
 ENV ARCH armv7l
 ENV HOSTCC gcc