Run as a container (#8) including Docker-in-Docker. (#84)

This adds a very simple Dockerfile and run script for running `act_runner` as a container.

It also allows setting `Privileged` and `ContainerOptions` flags via the new config file when spawning task containers.  The combination makes it possible to use Docker-in-Docker (which requires `privileged` mode) as well as pass any other options child Docker containers may require.

For example, if Gitea is running in Docker on the same machine, for the `checkout` action to behave as expected from a task container launched by `act_runner`, it might be necessary to map the hostname via something like:

```
container:
  network_mode: bridge
  privileged: true
  options: --add-host=my.gitea.hostname:host-gateway
```

> NOTE: Description updated to reflect latest code.
> NOTE: Description updated to reflect latest code (again).

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/84
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: Thomas E Lackey <telackey@bozemanpass.com>
Co-committed-by: Thomas E Lackey <telackey@bozemanpass.com>

run.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+
+if [[ ! -d /data ]]; then
+  mkdir -p /data
+fi
+
+cd /data
+
+CONFIG_ARG=""
+if [[ ! -z "${CONFIG_FILE}" ]]; then
+  CONFIG_ARG="--config ${CONFIG_FILE}"
+fi
+
+# Use the same ENV variable names as https://github.com/vegardit/docker-gitea-act-runner
+
+if [[ ! -s .runner ]]; then
+  try=$((try + 1))
+  success=0
+
+  # The point of this loop is to make it simple, when running both act_runner and gitea in docker,
+  # for the act_runner to wait a moment for gitea to become available before erroring out.  Within
+  # the context of a single docker-compose, something similar could be done via healthchecks, but
+  # this is more flexible.
+  while [[ $success -eq 0 ]] && [[ $try -lt ${GITEA_MAX_REG_ATTEMPTS:-10} ]]; do
+    act_runner register \
+      --instance "${GITEA_INSTANCE_URL}" \
+      --token    "${GITEA_RUNNER_REGISTRATION_TOKEN}" \
+      --name     "${GITEA_RUNNER_NAME:-`hostname`}" \
+      --labels   "${GITEA_RUNNER_LABELS}" \
+      ${CONFIG_ARG} --no-interactive > /tmp/reg.log 2>&1
+
+    cat /tmp/reg.log
+
+    cat /tmp/reg.log | grep 'Runner registered successfully' > /dev/null
+    if [[ $? -eq 0 ]]; then
+      echo "SUCCESS"
+      success=1
+    else
+      echo "Waiting to retry ..."
+      sleep 5
+    fi
+  done
+fi
+
+act_runner daemon ${CONFIG_ARG}