add tzdata package

Dockerfile

@@ -14,7 +14,7 @@ FROM alpine:3.23
 
 WORKDIR /root
 
-RUN apk add --no-cache ca-certificates && \
+RUN apk add --no-cache ca-certificates tzdata && \
   chmod a+rw /var/lock
 
 COPY --from=builder /app/cmd/backup/backup /usr/bin/backup

cmd/backup/command.go

@@ -36,6 +36,7 @@ func (c *command) runAsCommand() error {
 	}
 
 	for _, config := range configurations {
+		c.logStartupWarnings(config)
 		if err := runScript(config); err != nil {
 			return errwrap.Wrap(err, "error running script")
 		}
@@ -102,6 +103,8 @@ func (c *command) schedule(strategy configStrategy) error {
 
 	for _, cfg := range configurations {
 		config := cfg
+		c.logStartupWarnings(config)
+
 		id, err := c.cr.AddFunc(config.BackupCronExpression, func() {
 			c.logger.Info(
 				fmt.Sprintf(
@@ -150,3 +153,9 @@ func (c *command) must(err error) {
 		os.Exit(1)
 	}
 }
+
+func (c *command) logStartupWarnings(config *Config) {
+	for _, w := range config.timezoneDeprecationWarnings() {
+		c.logger.Warn(w)
+	}
+}

cmd/backup/config.go

@@ -4,6 +4,7 @@
 package main
 
 import (
+	"bufio"
 	"bytes"
 	"crypto/x509"
 	"encoding/pem"
@@ -97,6 +98,7 @@ type Config struct {
 	GoogleDriveImpersonateSubject        string          `split_words:"true"`
 	GoogleDriveEndpoint                  string          `split_words:"true"`
 	GoogleDriveTokenURL                  string          `split_words:"true"`
+	Timezone                             string          `envconfig:"TZ"`
 	source                               string
 	additionalEnvVars                    map[string]string
 }
@@ -322,3 +324,81 @@ func (c *Config) resolve() (reset func() error, warnings []string, err error) {
 	}
 	return
 }
+
+func mountedPaths(path string) (map[string]struct{}, error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer func() { _ = file.Close() }()
+
+	mounts := make(map[string]struct{})
+	scanner := bufio.NewScanner(file)
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		parts := strings.SplitN(line, " - ", 2)
+		fields := strings.Fields(parts[0])
+		if len(fields) < 5 {
+			continue
+		}
+		mounts[fields[4]] = struct{}{}
+	}
+
+	if err := scanner.Err(); err != nil {
+		return nil, err
+	}
+
+	return mounts, nil
+}
+
+func (c *Config) timezoneDeprecationWarnings() []string {
+	mounts, err := mountedPaths("/proc/self/mountinfo")
+	if err != nil {
+		return nil
+	}
+
+	deprecatedMounts := []string{
+		"/etc/timezone",
+		"/etc/localtime",
+		"/usr/share/zoneinfo",
+	}
+
+	var found []string
+	for _, mnt := range deprecatedMounts {
+		if _, ok := mounts[mnt]; ok {
+			found = append(found, mnt)
+		}
+	}
+
+	if len(found) == 0 {
+		return nil
+	}
+
+	var warnings []string
+
+	// Primary deprecation message (compressed)
+	warnings = append(warnings,
+		fmt.Sprintf(
+			"Deprecated timezone bind mounts detected: %s. Support for these will be removed in a future version.",
+			strings.Join(found, ", "),
+		),
+	)
+
+	// Guidance based on TZ usage
+	if c.Timezone == "" {
+		warnings = append(warnings,
+			"Set the container timezone using the `TZ` environment variable instead.",
+			"Refer to the documentation for migration details.",
+		)
+	} else {
+		warnings = append(warnings,
+			fmt.Sprintf(
+				"`TZ=%s` is set, but deprecated timezone bind mounts are still present. Remove the bind mounts after confirming timezone handling works as expected.",
+				c.Timezone,
+			),
+		)
+	}
+
+	return warnings
+}

cmd/backup/print_config.go

@@ -34,6 +34,9 @@ func runPrintConfig() error {
 			for _, warning := range warnings {
 				fmt.Printf("warning:%s\n", warning)
 			}
+			for _, warning := range config.timezoneDeprecationWarnings() {
+				fmt.Printf("warning:%s\n", warning)
+			}
 			// insert line breaks before each field name, assuming field names start with uppercase letters
 			formatted := formatter.ReplaceAllString(fmt.Sprintf("%+v", *config), "\n$1")
 			fmt.Printf("%s\n", formatted)

docs/.gitignore

@@ -1,2 +1,3 @@
 _site
 .jekyll-cache
+.DS_Store

docs/how-tos/set-container-timezone.md

@@ -7,20 +7,49 @@ nav_order: 8
 
 # Set the timezone the container runs in
 
-By default a container based on this image will run in the UTC timezone.
-As the image is designed to be as small as possible, additional timezone data is not included.
-In case you want to run your cron rules in your local timezone (respecting DST and similar), you can mount your Docker host's `/etc/timezone`, `/etc/localtime`, and `/usr/share/zoneinfo` in read-only mode:
+:warning: **Deprecation Warning**  
+The method described below (bind-mounting files from the host) is **deprecated**. Please use the `TZ` environment variable (which uses the `tzdata` package) instead.
+
+> ```yml
+> services:
+>   backup:
+>     image: offen/docker-volume-backup:v2
+>     volumes:
+>       - data:/backup/my-app-backup:ro
+>       - /etc/timezone:/etc/timezone:ro
+>       - /etc/localtime:/etc/localtime:ro
+>       - /usr/share/zoneinfo:/usr/share/zoneinfo:ro
+>
+> volumes:
+>   data:
+> ```
+
+## Recommended approach (using `TZ`)
+
+By default, the container runs in the UTC timezone. To configure a different timezone in a portable and reliable way, set the `TZ` environment variable to a valid [tz database time zone](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones):
 
 ```yml
 services:
   backup:
     image: offen/docker-volume-backup:v2
+    environment:
+      - TZ=Europe/Berlin
     volumes:
       - data:/backup/my-app-backup:ro
-      - /etc/timezone:/etc/timezone:ro
-      - /etc/localtime:/etc/localtime:ro
-      - /usr/share/zoneinfo:/usr/share/zoneinfo:ro
 
 volumes:
   data:
 ```
+
+## Notes
+
+This approach is preferred because it:
+
+- avoids dependency on host configuration  
+- works consistently across environments  
+
+### Compatibility
+
+- Bind-mounting timezone files will continue to work if `TZ` is not set.
+- If `TZ` is set, it takes precedence over any bind-mounted timezone configuration.
+- An invalid `TZ` value will cause the container to default to UTC.