image/tree: Don't limit name width if non tty

Previously when no terminal was attached the width was assumed to be 80.
This is too short for most image names which truncated the names when
output was redirect (for example to `grep`).

This disabled the name truncation if the terminal width can't be
determined.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

cli/command/image/tree.go

@@ -230,11 +230,10 @@ func printImageTree(outs command.Streams, view treeView) {
 	}
 
 	out := tui.NewOutput(outs.Out())
+	isTerm := out.IsTerminal()
+
 	_, width := out.GetTtySize()
-	if width == 0 {
-		width = 80
-	}
-	if width < 20 {
+	if isTerm && width < 20 {
 		width = 20
 	}
 
@@ -243,8 +242,6 @@ func printImageTree(outs command.Streams, view treeView) {
 	untaggedColor := out.Color(tui.ColorTertiary)
 	titleColor := out.Color(tui.ColorTitle)
 
-	isTerm := out.IsTerminal()
-
 	out.Println(generateLegend(out, width))
 
 	possibleChips := getPossibleChips(view)
@@ -340,25 +337,27 @@ func printImageTree(outs command.Streams, view treeView) {
 // to display their content.
 func adjustColumns(width uint, columns []imgColumn, images []topImage) []imgColumn {
 	nameWidth := int(width)
-	for idx, h := range columns {
-		if h.Width == 0 {
-			continue
+	if nameWidth > 0 {
+		for idx, h := range columns {
+			if h.Width == 0 {
+				continue
+			}
+			d := h.Width
+			if idx > 0 {
+				d += columnSpacing
+			}
+			// If the first column gets too short, remove remaining columns
+			if nameWidth-d < 12 {
+				columns = columns[:idx]
+				break
+			}
+			nameWidth -= d
 		}
-		d := h.Width
-		if idx > 0 {
-			d += columnSpacing
-		}
-		// If the first column gets too short, remove remaining columns
-		if nameWidth-d < 12 {
-			columns = columns[:idx]
-			break
-		}
-		nameWidth -= d
 	}
 
 	// Try to make the first column as narrow as possible
 	widest := widestFirstColumnValue(columns, images)
-	if nameWidth > widest {
+	if width == 0 || nameWidth > widest {
 		nameWidth = widest
 	}
 	columns[0].Width = nameWidth