circleci: run cargo tarpaulin in a machine image

This way we can specify the necessary Docker security options to let
Tarpaulin run while still getting the benefits of using a prebuilt
Docker image.

Author: mgeisler

.circleci/config.yml

@@ -1,13 +1,18 @@
 version: 2
 jobs:
   build:
-    docker:
-      - image: xd009642/tarpaulin
+    machine: true
     steps:
       - checkout
+      - run:
+          name: Pull xd009642/tarpaulin
+          command: docker pull xd009642/tarpaulin
       - run:
           name: Generate coverage report
-          command: cargo tarpaulin --out Xml
+          command: >-
+            docker run --security-opt seccomp=unconfined
+            -v $PWD:/volume xd009642/tarpaulin
+            cargo tarpaulin --out Xml
       - run:
           name: Upload to codecov.io
           command: bash <(curl -s https://codecov.io/bash) -Z -f cobertura.xml