ci: Fixing cross build part2. (thanos-io#525)

Signed-off-by: Bartek Plotka <bwplotka@gmail.com>

Author: bwplotka

.circleci/config.yml

@@ -36,6 +36,18 @@ jobs:
 
             make test
 
+  # Cross build is needed for publish_release but needs to be done outside of docker.
+  cross_build:
+    machine: true
+    working_directory: /home/circleci/.go_workspace/src/github.com/improbable-eng/thanos
+    steps:
+    - checkout
+    - run: make crossbuild
+    - persist_to_workspace:
+        root: .
+        paths:
+        - .build
+
   publish_master:
     docker:
       # Available from https://hub.docker.com/r/circleci/golang/
@@ -67,7 +79,6 @@ jobs:
       - run: echo 'export PATH=${HOME}/bin:${PATH}' >> ${BASH_ENV}
       - attach_workspace:
           at: .
-      - run: make crossbuild
       - run: make tarballs-release
       - store_artifacts:
           path: .tarballs
@@ -92,9 +103,18 @@ workflows:
         filters:
           branches:
             only: master
+    - cross_build:
+        requires:
+        - test
+        filters:
+          tags:
+            only: /^v[0-9]+(\.[0-9]+){2}(-.+|[^-.]*)$/
+          branches:
+            ignore: /.*/
     - publish_release:
         requires:
         - test
+        - cross_build
         filters:
           tags:
             only: /^v[0-9]+(\.[0-9]+){2}(-.+|[^-.]*)$/