feat: Parameterized DOCKER_METADATA_PR_HEAD_SHA to allow for PR-HEAD tagging (#41)

* Parameterized DOCKER_METADATA_PR_HEAD_SHA to allow for PR-HEAD tagging

By default, Github Action uses merge commits during PR events for the
(referenced via the `github.sha` variable). Some users find this
challenging, since the merge commit contains code from the target branch,
which is troublesome for long-running branches, merge-conflicts, and
a hotfix workflow for deploying one-off containers.

This allows users to configure docker metadata-action to compute the SHA
tags appropriately. Unfortunately the metadata-action does not compute
based on a checkout action of the code, so we have to tell it to use
the correct event SHA explicitly with this environment variable.

See https://github.com/docker/metadata-action#environment-variables

* Fix the datatype for the linter

Co-authored-by: Dan Miller <[email protected]>

---------

Co-authored-by: Nathanael Beisiegel <[email protected]>
Co-authored-by: Dan Miller <[email protected]>

Author: 3 people

action.yml

@@ -68,6 +68,10 @@ inputs:
   secret-files:
     description: "List of secret files to expose to the build (e.g., key=filename, MY_SECRET=./secret.txt)"
     required: false
+  docker-metadata-pr-head-sha:
+    description: "Set to `true` to tag images with the PR HEAD SHA instead of the merge commit SHA within pull requests."
+    required: false
+    default: "false"
 outputs:
   image:
     description: "Docker image name"
@@ -94,6 +98,14 @@ runs:
     - name: Docker meta
       id: meta
       uses: docker/metadata-action@v4
+      env:
+        # Annoyingly, docker computes the SHA tags independently of any checkout
+        # action. This option tells this action to use the PR HEAD SHA during a
+        # pull request. It will then use `github.event.pull_request.head.sha`
+        # instead of the merge commit (`github.sha`). This is useful for folks
+        # who want to avoid merge commits in their PRs (for hotfix workflows,
+        # tag predictability, builds on merge-conflicts, etc).
+        DOCKER_METADATA_PR_HEAD_SHA: ${{ inputs.docker-metadata-pr-head-sha }}
       with:
         # list of Docker images to use as base name for tags
         images: |
@@ -108,7 +120,7 @@ runs:
           type=semver,pattern={{major}}.{{minor}}
           type=semver,pattern={{major}}
           type=raw,value=latest,enable={{is_default_branch}}
-          type=sha,format=long          
+          type=sha,format=long
           ${{ inputs.tags }}
         labels: |
           org.opencontainers.image.source=https://github.com/${{ inputs.organization }}/${{ inputs.repository }}
@@ -133,7 +145,7 @@ runs:
       shell: bash
       id: buildx-context
       run: |
-        docker context create buildx-context || true 
+        docker context create buildx-context || true
 
     - name: Set up QEMU
       uses: docker/setup-qemu-action@v2