Add basic CI checks using GH actions

.github/golint/Dockerfile

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+FROM golang:1.12.6
+RUN go get golang.org/x/lint/golint

.github/main.workflow

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+workflow "Pull Request" {
+  # TODO(awg): This won't run checks on PRs from forks. For now that seems to be
+  # a limitation of GH actions.
+  on = "push"
+  resolves = [
+    "vet",
+    "lint",
+    "test",
+  ]
+}
+
+action "vet" {
+  uses = "docker://golang:1.12.6"
+  env = {
+    GOFLAGS = "-mod=vendor"
+  }
+  runs = ["go", "vet", "./..."]
+}
+
+action "lint" {
+  uses = "./.github/golint"
+  env = {
+    GOFLAGS = "-mod=vendor"
+  }
+  runs = ["sh", "-c", "golint -set_exit_status $(go list ./...)"]
+}
+
+action "test" {
+  uses = "docker://golang:1.12.6"
+  env = {
+    GOFLAGS = "-mod=vendor"
+  }
+  runs = ["go", "test", "-race", "-cover", "./..."]
+}