implement a custom action to run the build script.

Author: christopherfrieler

.github/actions/execute-script-action/.gitignore

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+node_modules/

.github/actions/execute-script-action/README.md

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+# execute-script-action
+This is a custom GitHub Action to execute a shell script.
+The action will take care of making it executable.
+
+## Build
+The action is built as a JavaScript action.
+See [Creating a JavaScript action](https://docs.github.com/en/actions/creating-actions/creating-a-javascript-action) for a detailed documentation of the setup.
+
+### TL;DR
+**Prerequisite:** Node.js 12.x including npm.\
+**Install dependencies:** `npm install`\
+**Compile [dist](dist):** `node node_modules/@vercel/ncc/dist/ncc/cli.js build index.js --license licenses.txt` (Note: dist needs to be committed.)

.github/actions/execute-script-action/action.yml

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+name: 'Execute Script'
+description: 'Execute a shell script.'
+inputs:
+  script:
+    description: 'script to execute'
+    required: true
+    default: 'build.sh'
+outputs:
+  exit-code:
+    description: 'exit-code of the script'
+runs:
+  using: 'node12'
+  main: 'dist/index.js'