Workaround setup for: https://github.com/mileszs/wicked_pdf/issues/860 (see my comment there)

wicked_pdf.rb

# ./config/initializers/wicked_pdf.rb
WICKED_PDF_HELPER_SERVER_PORT = 28601  # NOTE no reason

WickedPdf.config = {
  # Layout file to be used for all PDFs
  # (but can be overridden in `render :pdf` calls)
  # layout: 'pdf.html',
}.tap do |config|
  rack_up = caller.map(&:to_s).grep(/rack.*server/i).any?  # SEE https://stackoverflow.com/a/49598328/714287
  if rack_up && Rails.configuration.asset_host.blank?
    # NOTE otherwise, all the assets/packs will just load from `...asset_host`
    # NOTE we rely on another process with wh2p anyway, so this checks out
    spawn Rails.root.join(
            'app/assets/helpers/wkhtmltopdf_asset_pack_locating_server'
          ).to_s, '-p', WICKED_PDF_HELPER_SERVER_PORT.to_s,
          chdir: Dir.tmpdir

    # NOTE this requires the rails server to support concurrent requests
    class WickedPdf
      def pdf_from_html_file(filepath, options)
        pdf_from_url("http://localhost:#{WICKED_PDF_HELPER_SERVER_PORT}#{filepath}", options)
      end
    end

    # SEE https://github.com/mileszs/wicked_pdf/issues/779#issuecomment-932406821 : config[:disable_smart_shrinking] = true

    ENV['XDG_RUNTIME_DIR'] = '/tmp/runtime-root'  # NOTE (default) stupid wkhtmltopdf
    config[:log_level] = :warn
    config[:raise_on_all_errors] = true
  else
    config[:enable_local_file_access] = true
  end
end

wkhtmltopdf_asset_pack_locating_server.rb

#!/usr/bin/env ruby
# ./app/assets/helpers/wkhtmltopdf_asset_pack_locating_server

require 'net/http'
require 'uri'

require 'sinatra'

# NOTE the whole setup is only useful if the assets are served by
#      the web app server (which runs on the local port 80)
ASSET_PACK_SVC_URI = 'http://localhost'


get '*' do |path_s|
  if path_s.end_with? '.html'
    content_type 'text/html'
    File.read path_s
  else
    asset_pack_url = "#{ASSET_PACK_SVC_URI}#{path_s}"
    begin
      res = Net::HTTP.get_response(URI.parse asset_pack_url)
      content_type res.content_type
      status res.code
      res.body
    rescue Net::ReadTimeout
      502
    end
  end
end