Fix empty file when using compose config in case of smaller source files

"docker compose config --output out.yml" resulted an empty file
when the generated output was smaller than 4097 bytes.
bufio.Writer doesn't seem necessary since only one write operation will happen.
This way there is no need for a new bufio.Writer that could be flushed.

Thanks for @thaJeztah for the idea of using os.WriteFile

Issue #10121

Signed-off-by: Ákos Takács <takacs.akos@it-sziget.hu>

cmd/compose/convert.go

@@ -17,11 +17,9 @@
 package compose
 
 import (
-	"bufio"
 	"bytes"
 	"context"
 	"fmt"
-	"io"
 	"os"
 	"sort"
 	"strings"
@@ -139,15 +137,10 @@ func runConvert(ctx context.Context, streams api.Streams, backend api.Service, o
 		return nil
 	}
 
-	var out io.Writer = streams.Out()
 	if opts.Output != "" && len(content) > 0 {
-		file, err := os.Create(opts.Output)
-		if err != nil {
-			return err
-		}
-		out = bufio.NewWriter(file)
+		return os.WriteFile(opts.Output, content, 0o666)
 	}
-	_, err = fmt.Fprint(out, string(content))
+	_, err = fmt.Fprint(streams.Out(), string(content))
 	return err
 }