Added github action

Dockerfile

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+# Container image that runs your code
+FROM php:7.3-cli-alpine3.9
+
+COPY --from=composer:1 /usr/bin/composer /usr/local/bin/composer
+
+RUN /usr/local/bin/composer global require magento/magento-coding-standard:*
+RUN ~/.composer/vendor/bin/phpcs --config-set installed_paths ../../magento/magento-coding-standard/
+RUN echo memory_limit = -1 >> /usr/local/etc/php/conf.d/custom-memory.ini
+# Copies your code file from your action repository to the filesystem path `/` of the container
+COPY entrypoint.sh /entrypoint.sh
+
+# Code file to execute when the docker container starts up (`entrypoint.sh`)
+ENTRYPOINT ["/entrypoint.sh"]

README.md

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+# Coding Standard for Magento 2 docker action
+
+This action checks coding standard of magento 2 extensions.
+
+## Inputs
+
+## `error-severity`
+
+The severity of error to check. Default `"10"`.
+
+## Outputs
+
+## `errors`
+
+The test result
+
+## Example usage
+
+uses: actions/magento2-docker-action@v1
+with:
+  error-severity: '10'

action.yml

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+# action.yml
+name: 'Coding Standard for Magento 2'
+description: 'Github Action for magento 2'
+inputs:
+  error-severity:  # id of input
+    description: 'Error severity'
+    required: false
+    default: '10'
+outputs:
+  time: # id of output
+    description: 'The test result'
+runs:
+  using: 'docker'
+  image: 'Dockerfile'
+  args:
+    - ${{ inputs.error-severity }}

entrypoint.sh

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+#!/bin/sh -l
+
+cd $GITHUB_WORKSPACE
+sh -c "/root/.composer/vendor/bin/phpcs --standard=Magento2 --error-severity=10 --warning-severity=8 $GITHUB_WORKSPACE -s $*"