Parse start commands in wings

This would replace the need to parse the start commands in the entrypoint so exec could be used instead of eval.

As seen in the gist below I have tested what it would do to the ark start command

https://gist.githubusercontent.com/parkervcp/db4d2d7b23b48f9aa0eacff79e77d972/raw/377601cd74e4d9c19e58ba0a56086bba438165fe/gistfile1.txt

server/server.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"net/http"
 	"os"
+	"strconv"
 	"strings"
 	"sync"
 
@@ -143,13 +144,30 @@ func (s *Server) Context() context.Context {
 	return s.ctx
 }
 
+func parseInvocation(invocation string, envvars map[string]interface{}, memory int64, port int, ip string) (parsed string) {
+	// replace "{{" and "}}" with "${" and "}" respectively
+	invocation = strings.Replace(invocation, "{{", "${", -1)
+	invocation = strings.Replace(invocation, "}}", "}", -1)
+
+	// replaces ${varname} with varval
+	for varname, varval := range envvars {
+		invocation = strings.Replace(invocation, fmt.Sprintf("${%s}", varname), fmt.Sprint(varval), -1)
+	}
+
+	invocation = strings.Replace(invocation, "${SERVER_PORT}", strconv.Itoa(port), -1)
+	invocation = strings.Replace(invocation, "${SERVER_MEMORY}", strconv.Itoa(int(memory)), -1)
+	invocation = strings.Replace(invocation, "${SERVER_IP}", ip, -1)
+
+	return invocation
+}
+
 // Returns all of the environment variables that should be assigned to a running
 // server instance.
 func (s *Server) GetEnvironmentVariables() []string {
 	out := []string{
 		// TODO: allow this to be overridden by the user.
 		fmt.Sprintf("TZ=%s", config.Get().System.Timezone),
-		fmt.Sprintf("STARTUP=%s", s.Config().Invocation),
+		fmt.Sprintf("STARTUP=%s", parseInvocation(s.Config().Invocation, s.Config().EnvVars, s.MemoryLimit(), s.Config().Allocations.DefaultMapping.Port, s.Config().Allocations.DefaultMapping.Ip)),
 		fmt.Sprintf("SERVER_MEMORY=%d", s.MemoryLimit()),
 		fmt.Sprintf("SERVER_IP=%s", s.Config().Allocations.DefaultMapping.Ip),
 		fmt.Sprintf("SERVER_PORT=%d", s.Config().Allocations.DefaultMapping.Port),