Adding Docker Swarm secrets support

[sweco-semhul]

Adding support for Docker Swarm secrets as option for sensitive variables.

And as a better solution for this issue, when running in a Docker Swarm environment, #278

[myoung34]

Im ok with this if its in a new file secrets.sh

#!/usr/bin/env bash
set -euo pipefail

SECRETS_DIR="/run/secrets"

for secret_file in "$SECRETS_DIR"/*; do
  # Skip if no files matched
  [[ -e "$secret_file" ]] || continue

  secret_name=$(basename "$secret_file")
  env_var_name=$(echo "$secret_name" | tr '[:lower:]' '[:upper:]' | tr '-' '_')

  # Read the secret
  secret_value=$(cat "$secret_file")

  # Export it
  export "$env_var_name=$secret_value"

  echo "Loaded secret: $env_var_name"
done

echo "Docker Swarm secrets loaded."

Then in entrypoint.sh at the top: _DOCKER_SWARM_SECRETS_ENABLED=${DOCKER_SWARM_SECRETS_ENABLED:-false}

and somewhere furhter below:

if [[ "${_DOCKER_SWARM_SECRETS_ENABLED}" == "true" ]]; then
  echo "Docker Swarm secrets enabled — loading secrets..."
  /secrets.sh
fi

also dont forget to modify https://github.com/myoung34/docker-github-actions-runner/blob/master/Dockerfile#L22-L23 to add script.sh with +x but also mkdir -p /run/secrets as well somewhere