Allow to run Outline on a server with another Watchtower

See https://containrrr.dev/watchtower/running-multiple-instances/

`com.centurylinklabs.watchtower.enable` and `--label-enable` are kept for backward compatibility.

Closes #48.

src/server_manager/install_scripts/install_server.sh

@@ -304,6 +304,7 @@ function start_shadowbox() {
  local -ar docker_shadowbox_flags=(
  --name "${CONTAINER_NAME}" --restart always --net host
  --label 'com.centurylinklabs.watchtower.enable=true'
+ --label 'com.centurylinklabs.watchtower.scope=shadowbox'
  -v "${STATE_DIR}:${STATE_DIR}"
  -e "SB_STATE_DIR=${STATE_DIR}"
  -e "SB_API_PORT=${API_PORT}"
@@ -336,7 +337,7 @@ function start_watchtower() {
  -v /var/run/docker.sock:/var/run/docker.sock)
  # By itself, local messes up the return code.
  local STDERR_OUTPUT
- STDERR_OUTPUT="$(docker run -d "${docker_watchtower_flags[@]}" containrrr/watchtower --cleanup --label-enable --tlsverify --interval "${WATCHTOWER_REFRESH_SECONDS}" 2>&1 >/dev/null)" && return
+ STDERR_OUTPUT="$(docker run -d "${docker_watchtower_flags[@]}" containrrr/watchtower --cleanup --label-enable --scope=shadowbox --tlsverify --interval "${WATCHTOWER_REFRESH_SECONDS}" 2>&1 >/dev/null)" && return
  readonly STDERR_OUTPUT
  log_error "FAILED"
  if docker_container_exists watchtower; then