docker bridge gateway is not available during prebuilds

Bug description

In order to run xdebug in ddev, ddev needs the ip of docker's bridge gateway.

During prebuilds, ddev displays an error that the value cannot be determined.
(same is true to opening a workspace after "Skip prebuild" button was pressed, or in case prebuild are failing, and Gitpod workspace open in "prebuild mode" (didn't have a better way to describe it).

Steps to reproduce

1. Open any repo that runs a prebuild, and click "Skip prebuild" button
2. In Gitpod terminal, run docker network inspect bridge
3. Confirm that IPAM.Config.Gateway is missing.

Compare that with a regular workspace, without skipping prebuild, run that same command and you'll see IPAM.Config.Gateway is there.

[
    {
        "Name": "bridge",
        "Id": "73e2ac1b72a1b74a809f0c91c993adc1320ea8de248c189bcbd51c1a49a2edd7",
        "Created": "2022-04-26T15:08:15.608436783Z",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "172.17.0.0/16",
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {},
        "Options": {
            "com.docker.network.bridge.default_bridge": "true",
            "com.docker.network.bridge.enable_icc": "true",
            "com.docker.network.bridge.enable_ip_masquerade": "true",
            "com.docker.network.bridge.host_binding_ipv4": "0.0.0.0",
            "com.docker.network.bridge.name": "docker0",
            "com.docker.network.driver.mtu": "1440"
        },
        "Labels": {}
    }
]

Workspace affected

No response

Expected behavior

No response

Example repository

No response

Anything else?

cc: @jankeromnes, @rfay, @aledbf

Reproduce the issue by skipping a prebuild, opening a workspace in prebuild mode, or using a failing prebuild, then run docker network inspect bridge and compare it with a regular workspace. Trace the prebuild workspace setup that creates the Docker bridge network. Done means IPAM.Config.Gateway is available in the affected modes and ddev can obtain the gateway IP for xdebug.