Add GitHub Action to install and setup ko (#347)

action.yml

@@ -0,0 +1,43 @@
+name: 'Setup ko'
+description: 'Install and authorize ko'
+branding:
+  icon: box
+  color: green
+inputs:
+  version:
+    description: 'Version of ko to install (tip, latest-release, v0.8.2, etc.)'
+    required: true
+    default: 'latest-release'
+runs:
+  using: "composite"
+  steps:
+  - shell: bash
+    run: |
+      # Install ko:
+      # - if version is "tip", install from tip of main.
+      # - if version is "latest-release", look up latest release.
+      # - otherwise, install the specified version.
+      case "${{ inputs.version }}" in
+      tip)
+        echo "Installing ko using go get"
+        go get github.com/google/ko@main
+        ;;
+      latest-release)
+        tag=$(curl -s https://api.github.com/repos/google/ko/releases/latest | jq -r '.tag_name')
+        ;;
+      *)
+        tag="${{ inputs.version }}"
+      esac
+
+      if [[ ! -z ${tag} ]]; then
+        echo "Installing ko @ ${tag}"
+        curl -fsL https://github.com/google/ko/releases/download/${tag}/ko_${tag:1}_Linux_x86_64.tar.gz | sudo tar xzf - -C /usr/local/bin ko
+      fi
+
+      # NB: username doesn't seem to matter.
+      echo "Logging in to ghcr.io"
+      echo "${{ github.token }}" | ko login ghcr.io --username "placeholder" --password-stdin
+
+      # Set KO_DOCKER_REPO for future steps.
+      echo "KO_DOCKER_REPO=ghcr.io/${{ github.repository }}"
+      echo "KO_DOCKER_REPO=ghcr.io/${{ github.repository }}" >> $GITHUB_ENV