Add support for specifying --host via environment variable

This commit adds support for specifying the `--host` option via the `RESTIC_HOST` environment variable. This is done by extending option processing in `cmd_backup.go` and for `restic.SnapshotFilter` in `find.go`.

cmd/restic/cmd_backup.go

@@ -114,7 +114,7 @@ func init() {
 	f.BoolVar(&backupOptions.StdinCommand, "stdin-from-command", false, "interpret arguments as command to execute and store its stdout")
 	f.Var(&backupOptions.Tags, "tag", "add `tags` for the new snapshot in the format `tag[,tag,...]` (can be specified multiple times)")
 	f.UintVar(&backupOptions.ReadConcurrency, "read-concurrency", 0, "read `n` files concurrently (default: $RESTIC_READ_CONCURRENCY or 2)")
-	f.StringVarP(&backupOptions.Host, "host", "H", "", "set the `hostname` for the snapshot manually. To prevent an expensive rescan use the \"parent\" flag")
+	f.StringVarP(&backupOptions.Host, "host", "H", "", "set the `hostname` for the snapshot manually (default: $RESTIC_HOST). To prevent an expensive rescan use the \"parent\" flag")
 	f.StringVar(&backupOptions.Host, "hostname", "", "set the `hostname` for the snapshot manually")
 	err := f.MarkDeprecated("hostname", "use --host")
 	if err != nil {
@@ -137,6 +137,11 @@ func init() {
 	// parse read concurrency from env, on error the default value will be used
 	readConcurrency, _ := strconv.ParseUint(os.Getenv("RESTIC_READ_CONCURRENCY"), 10, 32)
 	backupOptions.ReadConcurrency = uint(readConcurrency)
+
+	// parse host from env, if not exists or empty the default value will be used
+	if host := os.Getenv("RESTIC_HOST"); host != "" {
+		backupOptions.Host = host
+	}
 }
 
 // filterExisting returns a slice of all existing items, or an error if no