LocalWithDocker.conf

backend {
  # Override the default backend.
  #default = "LocalWithDocker"

  # The list of providers.
  providers {

    # The local provider is included by default in the reference.conf. This is an example.

    # Define a new backend provider.
    LocalWithDocker {
      # The actor that runs the backend. In this case, it's the Shared File System (SFS) ConfigBackend.
      actor-factory = "cromwell.backend.impl.sfs.config.ConfigBackendLifecycleActorFactory"

      # The backend custom configuration.
      config {

        # Optional limits on the number of concurrent jobs
        #concurrent-job-limit = 5

        # The list of possible runtime custom attributes.
        runtime-attributes = """
        String? docker
        String? docker_user
        String? cpu
        String? memory_mb
        """

        # Submit string when there is no "docker" runtime attribute.
        submit = "/usr/bin/env bash ${script}"

        # Submit string when there is a "docker" runtime attribute.
        submit-docker = """
        docker run \
          --rm -i \
          ${"--cpus=" + cpu} \
          ${"--memory=" + memory_mb + "m" } \
          ${"--user " + docker_user} \
          --entrypoint ${job_shell} \
          -v ${cwd}:${docker_cwd} \
          ${docker} ${script}
        """

        # Root directory where Cromwell writes job results.  This directory must be
        # visible and writeable by the Cromwell process as well as the jobs that Cromwell
        # launches.
        root = "cromwell-executions"

        # Root directory where Cromwell writes job results in the container. This value
        # can be used to specify where the execution folder is mounted in the container.
        # it is used for the construction of the docker_cwd string in the submit-docker
        # value above.
        dockerRoot = "/cromwell-executions"

        # File system configuration.
        filesystems {

          # For SFS backends, the "local" configuration specifies how files are handled.
          local {

            # Try to hard link (ln), then soft-link (ln -s), and if both fail, then copy the files.
            localization: [
              "hard-link", "soft-link", "copy"
            ]

            # Call caching strategies
            caching {
              # When copying a cached result, what type of file duplication should occur. Attempted in the order listed below:
              duplication-strategy: [
                "hard-link", "soft-link", "copy"
              ]

              # Possible values: file, path
              # "file" will compute an md5 hash of the file content.
              # "path" will compute an md5 hash of the file path. This strategy will only be effective if the duplication-strategy (above) is set to "soft-link",
              # in order to allow for the original file path to be hashed.
              hashing-strategy: "file"

              # When true, will check if a sibling file with the same name and the .md5 extension exists, and if it does, use the content of this file as a hash.
              # If false or the md5 does not exist, will proceed with the above-defined hashing strategy.
              check-sibling-md5: false
            }
          }
        }

        # The defaults for runtime attributes if not provided.
        default-runtime-attributes {
          failOnStderr: false
          continueOnReturnCode: 0
          cpu: 1
          memory: "4 MB"
        }
      }
    }
  }
}

yes! thank you for noticing -- fixed