Parsing JSON in go

Your code just needs to be modified to further unpackage the data. You're leaving it with each container object represented as an interface{} which makes it so you can't access the fields within it. Another assert on each to make it a map[string]interface{} will allow you to access the fields within by name (like IPv4Address). Here's a working example; https://play.golang.org/p/4OC5axN4Gd

Here's the important code change;

    containers := (foo[0]["Containers"]).(map[string]interface{})

    //fmt.Printf("containers %+v", containers)
    for _, v := range containers {
        unwrapped := v.(map[string]interface{})
        fmt.Printf("\ncont %+v\n", unwrapped["IPv4Address"])            
    }

v is just an interface{} so you have no way of accessing the IPv4Address field without asserting again/converting into a type.

Now that works fine, however, I would recommend getting away from the heavy use of interfaces in your code. Your code is full of unsafe operations that create a need for a lot of error handling (like every time you attempt access into the map you have the potential to throw, as well as anytime you do a type assert). There are plenty of examples of how to do this here on SO but if you leave a comment requesting it I can produce another example that will unmarshal into structs. This is a lot more safe and maintainable imo because most errors will be raised on the call to unmarshal, rather than having potentially a lot of code that works with the results that could all cause a panic.

Here is some play code that has all the types in it and uses unmarshal; https://play.golang.org/p/VfHst9GaiR

Your types can be represented with the following;

type DockerInstance struct {
    Name string `json:"Name"`
    Id string `json:"Id"`
    Scope string `json:"Scope"`
    Driver string `json:"Driver"`   
    EnableIPv6 bool `json:"EnableIPv6"`
    IPAM IPAM `json:"IPAM"`
    Internal bool `json:"Internal"`
    Containers map[string]Container `json:"Containers"`
    Options map[string]string `json:"Options"`
    Labels interface{} `json:"Labels"`
}

type IPAM struct {
    Driver string `json:"Driver"`
    Options interface{} `json:"Options"`
    Config []Conf `json:"Config"`
}

type Conf struct {
    Subnet string `json:"Subnet"`
}

type Container struct {
    Name string `json:"Name"`
        EndPointID string `json:"EndpointID"`
    MacAddress string `json:"MacAddress"`
        IPv4Address string `json:"IPv4Address"`
    IPv6Address string `json:"IPv6Address"`
}

There's a few stuff I still have set to interface, that's just cause the sample json doesn't include any data for them so I can't know what their type should be in the Go source. Here's some basic stuff I also added in your main to test out the objects/make sure I had things defined correctly;

docks := []DockerInstance{} //note you're using an array of the top level type
err := json.Unmarshal([]byte(teststring), &docks)
if err != nil {
    fmt.Println(err)
} else {
    fmt.Println()
    fmt.Println()
    fmt.Println(docks[0].Name)
}

for k, v := range docks[0].Containers {
    fmt.Println(k)
    fmt.Println(v.IPv4Address)
}

for k, v := range docks[0].Options {
      fmt.Println(k)
    fmt.Println(v)
}