Adds Packer action (#1)

* adds `Dockerfile` and `entrypoint.sh`

* adds GitHub Action definition

* adds `README.md`

Dockerfile

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+FROM hashicorp/packer:light@sha256:df7feeff930b04a42f2027dd0924392246f7b5a38f0c56531a2d14cd0d1e9408
+
+COPY "entrypoint.sh" "/entrypoint.sh"
+
+ENTRYPOINT ["/entrypoint.sh"]

README.md

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+# GitHub Action: Packer
+
+> GitHub Action for running Packer [commands](https://packer.io/docs/commands/index.html).
+
+## Table of Contents
+
+- [GitHub Action: Packer](#github-action-packer)
+  - [Table of Contents](#table-of-contents)
+  - [Usage](#usage)
+    - [Inputs](#inputs)
+      - [command](#command)
+      - [arguments](#arguments)
+      - [target](#target)
+  - [Notes](#notes)
+  - [Author Information](#author-information)
+  - [License](#license)
+
+## Usage
+
+Add the Action to your [GitHub Workflow](https://help.github.com/en/actions/configuring-and-managing-workflows/configuring-a-workflow#creating-a-workflow-file) like so:
+
+```yaml
+---
+
+name: Packer
+
+on:
+  push:
+
+jobs:
+  packer:
+    runs-on: ubuntu-latest
+    name: packer
+
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v2
+
+      # fix backwards incompatibilities in template
+      - name: Fix Template
+        uses: operatehappy/packer-github-action
+        with:
+          command: fix
+
+      # validate templates
+      - name: Validate Template
+        uses: operatehappy/packer-github-action
+        with:
+          command: validate
+          arguments: -syntax-only
+          target: artifacts.pkr.json packer.json
+
+      # build artifact
+      - name: Build Artifact
+        uses: operatehappy/packer-github-action
+        with:
+          command: build
+          arguments: "-color=false -on-error=abort"
+
+      # additional steps to process artifacts
+```
+
+### Inputs
+
+| Name        | Description           | Required | Default              |
+|-------------|-----------------------|----------|----------------------|
+| `command`   | command to execute    | yes      |                      |
+| `arguments` | arguments for command | no       |                      |
+| `target`    | file(s) to target     | yes      | `artifacts.pkr.json` |
+
+#### `command`
+
+`command` supports the following subset of Packer [CLI commands](https://packer.io/docs/commands/index.html):
+
+- [build](https://packer.io/docs/commands/build.html) to generate a set of artifacts from a template
+- [fix](https://packer.io/docs/commands/fix.html) to find and correct backwards incompatible stanzas in a template
+- [validate](https://packer.io/docs/commands/validate.html) to validate the syntax and configuration of a template
+
+Other CLI commands (`console`, `inspect` and `version`) are _technically_ supported, but have limited utility in a GitHub Actions Workflow, due to their interactive nature.
+
+#### `arguments`
+
+`arguments` supports all options available for the respective `command`.
+
+The arguments must be provided as a single string. Multiple arguments should be concatenated like so: "-color=false -on-error=abort"
+
+#### `target`
+
+`target` supports a string consisting of one or more file paths:
+
+ ```yaml
+    # single file
+    target: artifacts.pkr.json
+
+    # multiple files, separated by whitespace
+    target: artifacts.pkr.json packer.json
+```
+
+ The Action will iterate over each file and run each `command`, separately.
+
+## Notes
+
+- To enable debug logging, create a secret named `ACTIONS_STEP_DEBUG` with the value `true`. See [here](https://help.github.com/en/actions/reference/workflow-commands-for-github-actions#setting-a-debug-message) for more information.
+
+## Author Information
+
+This module is maintained by the contributors listed on [GitHub](https://github.com/operatehappy/packer-github-action/graphs/contributors).
+
+Development of this module was sponsored by [Operate Happy](https://github.com/operatehappy).
+
+## License
+
+Licensed under the Apache License, Version 2.0 (the "License").
+
+You may obtain a copy of the License at [apache.org/licenses/LICENSE-2.0](http://www.apache.org/licenses/LICENSE-2.0).
+
+Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an _"AS IS"_ basis, without WARRANTIES or conditions of any kind, either express or implied.
+
+See the License for the specific language governing permissions and limitations under the License.

action.yml

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+---
+
+name: Packer GitHub Actions
+author: Kerim Satirli <kerim@operatehappy.com>
+description: Run Packer commands
+
+inputs:
+  command:
+    description: command to execute
+    required: true
+  arguments:
+    description: arguments for command
+  target:
+    description: file(s) to target
+    required: true
+    default: artifacts.pkr.json
+
+outputs:
+  command:
+    description: interpolated command (incl. arguments) that will be executed
+
+runs:
+  using: 'docker'
+  image: 'Dockerfile'
+
+branding:
+  color: blue
+  icon: package

entrypoint.sh

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+#!/usr/bin/env bash
+
+# fail if INPUT_COMMAND is not set
+if [ -z "${INPUT_COMMAND}" ]; then
+  echo "Required variable \`command\` is missing"
+  exit 1
+fi
+
+# assemble operation
+if [ -z "${INPUT_ARGUMENTS}" ]; then
+  OPERATION="packer ${INPUT_COMMAND}"
+else
+  OPERATION="packer ${INPUT_COMMAND} ${INPUT_ARGUMENTS}"
+fi
+
+echo "::debug:: Executing command: ${OPERATION}"
+
+# cast INPUT_TARGET string to "array"
+# shellcheck disable=SC2206
+TARGETS=(${INPUT_TARGET})
+
+# iterate over target(s)
+for TARGET in "${TARGETS[@]}"; do
+  echo "::debug:: Processing target ${TARGET}"
+
+  # shellcheck disable=SC2086
+  ${OPERATION} "${TARGET}"
+done