Refactor environment handling logic to separate a server from the environment handler itself

This change makes the environment handling logic execute independent of the server itself and should make it much easier for people to contribute changes and additional environment handlers down the road without polluting the server object even more.

There is still a lot of work to do on this front to make things easier to work with, and there are some questionable design decisions at play I'm sure.

Welcome to additional modifications and cleanup to make this code easier to reason about and work with.

environment/allocations.go

@@ -0,0 +1,67 @@
+package environment
+
+import (
+	"fmt"
+	"github.com/docker/go-connections/nat"
+	"strconv"
+)
+
+// Defines the allocations available for a given server. When using the Docker environment
+// driver these correspond to mappings for the container that allow external connections.
+type Allocations struct {
+	// Defines the default allocation that should be used for this server. This is
+	// what will be used for {SERVER_IP} and {SERVER_PORT} when modifying configuration
+	// files or the startup arguments for a server.
+	DefaultMapping struct {
+		Ip   string `json:"ip"`
+		Port int    `json:"port"`
+	} `json:"default"`
+
+	// Mappings contains all of the ports that should be assigned to a given server
+	// attached to the IP they correspond to.
+	Mappings map[string][]int `json:"mappings"`
+}
+
+// Converts the server allocation mappings into a format that can be understood by Docker. While
+// we do strive to support multiple environments, using Docker's standardized format for the
+// bindings certainly makes life a little easier for managing things.
+func (a *Allocations) Bindings() nat.PortMap {
+	var out = nat.PortMap{}
+
+	for ip, ports := range a.Mappings {
+		for _, port := range ports {
+			// Skip over invalid ports.
+			if port < 1 || port > 65535 {
+				continue
+			}
+
+			binding := []nat.PortBinding{
+				{
+					HostIP:   ip,
+					HostPort: strconv.Itoa(port),
+				},
+			}
+
+			out[nat.Port(fmt.Sprintf("%d/tcp", port))] = binding
+			out[nat.Port(fmt.Sprintf("%d/udp", port))] = binding
+		}
+	}
+
+	return out
+}
+
+// Converts the server allocation mappings into a PortSet that can be understood
+// by Docker. This formatting is slightly different than "Bindings" as it should
+// return an empty struct rather than a binding.
+//
+// To accomplish this, we'll just get the values from "Bindings" and then set them
+// to empty structs. Because why not.
+func (a *Allocations) Exposed() nat.PortSet {
+	var out = nat.PortSet{}
+
+	for port := range a.Bindings() {
+		out[port] = struct{}{}
+	}
+
+	return out
+}