Add Circle CI runes

Advantage over Travis for this usecase is the automatic artifact handling
without needing to mess with S3 or similar, making it simple to offer
downloads.

Signed-off-by: Ian Campbell <ijc@docker.com>

.circleci/config.yml

@@ -0,0 +1,55 @@
+version: 2
+jobs:
+  build:
+    working_directory: /go/src/github.com/linuxkit/linuxkit
+    docker:
+      - image: circleci/golang:1.9-stretch
+    steps:
+      - checkout
+      - run: mkdir -p ./bin
+      - run:
+          name: Versions
+          command: |
+            set -x
+            go version
+            cat /etc/os-release
+      - run:
+          name: Dependencies
+          command: |
+            go get -u github.com/golang/lint/golint
+            go get -u github.com/gordonklaus/ineffassign
+      - run:
+          name: Lint
+          command: make local-check
+      - run:
+          name: Build amd64/linux
+          environment:
+            GOOS: linux
+            GOARCH: amd64
+          command: make LOCAL_TARGET=bin/linuxkit-$GOOS-$GOARCH local-build
+      - run:
+          name: Build arm64/linux
+          environment:
+            GOOS: linux
+            GOARCH: arm64
+          command: make LOCAL_TARGET=bin/linuxkit-$GOOS-$GOARCH local-build
+      - run:
+          name: Build amd64/darwin
+          environment:
+            GOOS: darwin
+            GOARCH: amd64
+          command: make LOCAL_TARGET=bin/linuxkit-$GOOS-$GOARCH local-build
+      - run:
+          name: Build amd64/windows
+          environment:
+            GOOS: windows
+            GOARCH: amd64
+          command: make LOCAL_TARGET=bin/linuxkit-$GOOS-$GOARCH local-build
+      - run:
+          name: Test
+          command: make local-test
+      - run:
+          name: Checksum
+          command: cd bin && sha256sum linuxkit-*-* > SHA256SUM
+      - store_artifacts:
+          path: ./bin