Improve platform support

action.yml

@@ -21,8 +21,27 @@ inputs:
 runs:
   using: composite
   steps:
-    - run: bash --noprofile --norc "${GITHUB_ACTION_PATH:?}/main.sh"
-      shell: bash
+    - run: |
+        set -eu
+        if ! command -v bash >/dev/null; then
+          if grep -Eq '^ID=alpine' /etc/os-release; then
+            printf '::group::Install packages required for checkout-action (bash)\n'
+            # NB: sync with apk_install in main.sh
+            if command -v sudo >/dev/null; then
+              sudo apk --no-cache add bash
+            elif command -v doas >/dev/null; then
+              doas apk --no-cache add bash
+            else
+              apk --no-cache add bash
+            fi
+            printf '::endgroup::\n'
+          else
+            printf '::error::checkout-action requires bash\n'
+            exit 1
+          fi
+        fi
+        bash --noprofile --norc "${GITHUB_ACTION_PATH:?}/main.sh"
+      shell: sh
       env:
         INPUT_TOOL: ${{ inputs.tool }}
         INPUT_CHECKSUM: ${{ inputs.checksum }}