add hello-world action

as per https://docs.github.com/en/actions/sharing-automations/creating-actions/creating-a-docker-container-action

.hadolint.yaml

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+ignored:
+  - DL3008

Dockerfile

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+# Container image that runs your code
+FROM debian:12.8-slim
+
+# compare https://kubernetes.io/docs/tasks/tools/install-kubectl-linux/
+RUN apt-get update && \
+  export DEBIAN_FRONTEND=noninteractive && \
+  apt-get install --no-install-recommends -y git nodejs gpg npm dnsutils \
+  apt-transport-https ca-certificates curl gnupg # packages in this line required for kubectl && \
+  curl -fsSL https://pkgs.k8s.io/core:/stable:/v1.32/deb/Release.key | gpg --dearmor -o /etc/apt/keyrings/kubernetes-apt-keyring.gpg && \
+  chmod 644 /etc/apt/keyrings/kubernetes-apt-keyring.gpg && \
+  echo 'deb [signed-by=/etc/apt/keyrings/kubernetes-apt-keyring.gpg] https://pkgs.k8s.io/core:/stable:/v1.32/deb/ /' | tee /etc/apt/sources.list.d/kubernetes.list && \
+  chmod 644 /etc/apt/sources.list.d/kubernetes.list   # helps tools such as command-not-found to work correctly && \
+  apt-get update && apt-get install -y kubectl && \
+  apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
+
+# 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: "Hello World"
+description: "Greet someone and record the time"
+inputs:
+  who-to-greet: # id of input
+    description: "Who to greet"
+    required: true
+    default: "World"
+outputs:
+  time: # id of output
+    description: "The time we greeted you"
+runs:
+  using: "docker"
+  image: "Dockerfile"
+  args:
+    - ${{ inputs.who-to-greet }}

entrypoint.sh

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+#!/bin/sh -l
+
+echo "Hello $1"
+time=$(date)
+echo "time=$time" >>"$GITHUB_OUTPUT"