Add workflow for docker

Author: ribose-jeffreylau

Diff for: .github/workflows/docker.yml

@@ -0,0 +1,28 @@
+name: Docker
+
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+      - 'release/**'
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    if: "!contains(github.event.head_commit.message, 'skip ci')"
+    strategy:
+      matrix:
+        env:
+          - SCRIPT: docker_images
+
+    env: ${{ matrix.env }}
+    timeout-minutes: 70
+    continue-on-error: true
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: Build Docker images
+        run: |
+          bash -x "./examples/${SCRIPT}.sh"

Diff for: README.adoc

@@ -52,6 +52,7 @@ A set of build scripts for GNU Privacy Guard.
 |image:https://github.com/rnpgp/gpg-build-scripts/actions/workflows/macos-10.15-head.yml/badge.svg["macOS 10.15 (head)", link="https://github.com/rnpgp/gpg-build-scripts/actions/workflows/macos-10.15-head.yml"]
 |==========================
 
+image:https://github.com/rnpgp/gpg-build-scripts/actions/workflows/docker.yml/badge.svg["Docker", link="https://github.com/rnpgp/gpg-build-scripts/actions/workflows/docker.yml"]
 
 == Prerequisites