docs: Github action SDD proposal

docs/flank-github-action/flank_github_acton_sdd.md

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+# Flank Github action
+
+Add Github action which allows running Flank
+
+# References
+
+- [Github actions documentation](https://github.com/features/actions)
+
+- [Creating a Docker container action](https://docs.github.com/en/actions/creating-actions/creating-a-docker-container-action)
+
+- [Creating a JavaScript action](https://docs.github.com/en/actions/creating-actions/creating-a-javascript-action)
+
+- [Creating a composite run steps action](https://docs.github.com/en/actions/creating-actions/creating-a-composite-run-steps-action)
+
+- [Github actions marketplace](https://github.com/marketplace?type=actions)
+
+
+
+# Motivation
+
+Bitrise and Circle CI have steps in their CIs to run Flank. As Github is a widely used code repository and Github actions become more and more popular there should be also Github action that allows running Flank. 
+
+After creating Github action, Flank could reach more users.
+
+# Goals
+
+- Flank actions should be as easy as possible to run
+- Flank action should be maintainable together with Flank runner (always be up to date with options)
+- Flank action should be available on the public repository
+
+# Design
+
+There should be a possibility to specify all flank options using Github action variables as input, as well as passing the configuration itself.
+
+User must provide at least:
+
+- Flank version (default latest)
+- platform to run (iOS or Android)
+- Flank service account file to authenticate
+- Flank options or flank configuration file path
+
+# API
+
+Flank Github action will be developed using [composite run steps action](https://docs.github.com/en/actions/creating-actions/creating-a-composite-run-steps-action) which just packs other actions and runs it as a single action.
+
+### Design proposal
+
+```yaml
+name: 'Flank'
+description: 'Run Flank from Github actions!'
+inputs:
+	version: 
+		description: 'Version of flank to run'
+		required: true
+		default: <latest>
+	platform: 
+		description: 'Platform to run iOS or Android'
+		required: true
+  flank_option1:
+    description: 'Description of option 1'
+    required: false # validation will done when running
+    ...
+  flank_optionX:
+    description: 'Description of option X'
+    required: false # validation will done when running
+  flank_configuration_file:
+    description: 'Path to configuration file'
+    required: false # validation will done when running
+	flank_service_account: 
+	  description: 'Path to service account file to authenticate'
+    required: true
+outputs:
+  output_report:
+    description: "Output report"
+    value: ${{ steps.report.outputs.random-id }}
+runs:
+  using: "composite"
+  steps:
+    - id: download flank
+      run: <download flank>
+      shell: bash
+    - id: validate configuration
+      run: <check if configuation file or any options are specified>
+      shell: bash
+    - id: run flank
+      run: <check if configuation file or any options are specified>
+      shell: bash
+    - id: report
+      run: <show run report to user>
+      shell: bash
+```
+
+
+
+### Usage
+
+##### With config file
+
+```yaml
+on: [push]
+
+jobs:
+  hello_world_job:
+    runs-on: ubuntu-latest
+    name: A job to say hello
+    steps:
+    - uses: actions/checkout@v2
+    - id: flank
+      uses: actions/flank@v1
+      with:
+        version: '21.01.0'
+        platform: 'android'
+        flank_service_account: './service_account.json'
+        flank_configuration_file: './flank.yml'
+    - run: cat ${{ steps.flank.outputs.output_report }}
+      shell: bash
+```
+
+##### With options
+
+```yaml
+on: [push]
+
+jobs:
+  hello_world_job:
+    runs-on: ubuntu-latest
+    name: A job to say hello
+    steps:
+    - uses: actions/checkout@v2
+    - id: flank
+      uses: actions/flank@v1
+      with:
+        version: '21.01.0'
+        platform: 'android'
+        flank_service_account: './service_account.json'
+				app: "../test_projects/android/apks/app-debug.apk"
+  			test: "../test_projects/android/apks/app-debug-androidTest.apk"
+  			results-dir: test_dir
+				legacy-junit-result: true
+    - run: cat ${{ steps.flank.outputs.output_report }}
+      shell: bash
+```
+
+
+
+# Results
+
+After finishing Flank GitHub actions should be published to [Github actions marketplace](https://github.com/marketplace?type=actions). 
+
+There should be also an announcement on Flank channel on Slack.
+
+The thing to consider is also using it for our internal verification together with integration tests
+
+# Dependencies
+
+Github action needs to have `action.yml` file in the root of the repository, however, Flank repository has the file for posting Slack message after release. However it is only used by Flank team, so it will be best to move it as a side repository and keep the main Flank action in Flank's mono repository
+
+# Testing
+
+A new testing repository will be set up to test action as described in [GitHub actions documentation](https://docs.github.com/en/actions/creating-actions/creating-a-composite-run-steps-action#testing-out-your-action-in-a-workflow)
+
+# Alternatives Considered
+
+The investigation was done to choose the best way to develop custom GitHub action.
+
+[Docker container action](https://docs.github.com/en/actions/creating-actions/creating-a-docker-container-action) is not the best choice, because users are [forced to run Flank action on Ubuntu workflow](https://docs.github.com/en/actions/creating-actions/about-actions#types-of-actions).
+
+The second alternative considered was using [JavaScript as action language](https://docs.github.com/en/actions/creating-actions/creating-a-javascript-action). However, this is another language to maintain in our project and it will not be the best option to choose for such an important thing