.github/workflows: Add a unit test workflow that mirrors the ci/prow/unit job

Add a GH action that emulates the ci/prow/unit check where it installs
the local dependencies (skopeo/kubebuilder) and runs the `make unit`
target.

Signed-off-by: timflannagan <[email protected]>

Author: timflannagan

.github/workflows/unit.yml

@@ -0,0 +1,27 @@
+name: unit
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    paths:
+      - '**'
+      - '!doc/**'
+jobs:
+  unit:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-go@v2
+        with:
+          go-version: '~1.16'
+      - name: Install Kubebuilder
+        run: |
+          os=$(go env GOOS)
+          arch=$(go env GOARCH)
+          curl -L https://go.kubebuilder.io/dl/2.3.1/${os}/${arch} | tar -xz -C /tmp/
+          sudo mv /tmp/kubebuilder_2.3.1_${os}_${arch} /usr/local/kubebuilder
+      - name: (debug) Check if skopeo is installed
+        run: skopeo --version
+      - name: Run unit tests
+        run: make unit