feat: example docker action

Dockerfile

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+# Container image that runs your code
+FROM alpine:3.10
+
+# 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"]

action.yml

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+# action.yml
+name: 'Test Docker'
+description: 'Test Docker'
+runs:
+  using: 'docker'
+  image: 'Dockerfile'

entrypoint.sh

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+#!/bin/sh -l
+
+sleep 60