Allow to specify the server Timezone at egg level (#21)

* Allow to specify the server Timezone at egg level

server/server.go

@@ -6,10 +6,11 @@ import (
 	"net/http"
 	"os"
 	"path"
+	"regexp"
 	"strconv"
 	"strings"
 	"sync"
-	"regexp"
+	"time"
 
 	"emperror.dev/errors"
 	"github.com/apex/log"
@@ -146,6 +147,25 @@ func (s *Server) Context() context.Context {
 	return s.ctx
 }
 
+// DetermineServerTimezone checks the envvars for a non-empty SERVER_TIMEZONE key,
+// validates if it's a valid timezone, and returns it. If not, returns the defaultTimezone.
+func DetermineServerTimezone(envvars map[string]interface{}, defaultTimezone string) string {
+	// Check if SERVER_TIMEZONE exists in envvars and is non-empty
+	timezone, ok := envvars["SERVER_TIMEZONE"].(string)
+	if ok && timezone != "" {
+		// Validate the timezone
+		_, err := time.LoadLocation(timezone)
+		if err == nil {
+			// Valid timezone, return it
+			return timezone
+		}
+		// Invalid timezone, fall through to return defaultTimezone
+	}
+
+	// Return the defaultTimezone if SERVER_TIMEZONE is not set, empty, or invalid
+	return defaultTimezone
+}
+
 // parseInvocation parses the start command in the same way we already do in the entrypoint
 // We can use this to set the container command with all variables replaced.
 func parseInvocation(invocation string, envvars map[string]interface{}, memory int64, port int, ip string) (parsed string) {
@@ -191,8 +211,7 @@ func parseInvocation(invocation string, envvars map[string]interface{}, memory i
 // 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("TZ=%s", DetermineServerTimezone(s.Config().EnvVars, config.Get().System.Timezone)),
 		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),