Fix very minor continuation bugs for better coverage

There were some very minor/subtle bugs in how I implemented continuation that wouldn't affect any real-world parsing we did, but still bothered me because I'm me.  This fixes them (and further increases test coverage as a result).

pkg/dockerfile/parse.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"strconv"
 	"strings"
+	"unicode"
 )
 
 type Metadata struct {
@@ -31,10 +32,11 @@ func ParseReader(dockerfile io.Reader) (*Metadata, error) {
 		line := strings.TrimSpace(scanner.Text())
 
 		if line == "" {
-			// ignore blank lines
+			// ignore straight up blank lines (no complexity)
 			continue
 		}
 
+		// (we can't have a comment that ends in a continuation line - that's not continuation, that's part of the comment)
 		if line[0] == '#' {
 			// TODO handle "escape" parser directive
 			// TODO handle "syntax" parser directive -- explode appropriately (since custom syntax invalidates our Dockerfile parsing)
@@ -44,20 +46,36 @@ func ParseReader(dockerfile io.Reader) (*Metadata, error) {
 
 		// handle line continuations
 		// (TODO see note above regarding "escape" parser directive)
-		for line[len(line)-1] == '\\' && scanner.Scan() {
-			nextLine := strings.TrimSpace(scanner.Text())
-			if nextLine == "" || nextLine[0] == '#' {
-				// ignore blank lines and comments
+		for line[len(line)-1] == '\\' {
+			if !scanner.Scan() {
+				line = line[0:len(line)-1]
+				break
+			}
+			// "strings.TrimRightFunc(IsSpace)" because whitespace *after* the escape character is supported and ignored 🙈
+			nextLine := strings.TrimRightFunc(scanner.Text(), unicode.IsSpace)
+			if nextLine == "" { // if it's all space, TrimRight will be TrimSpace 😏
+				// ignore "empty continuation" lines (https://github.com/moby/moby/pull/33719)
+				continue
+			}
+			if strings.TrimLeftFunc(nextLine, unicode.IsSpace)[0] == '#' {
+				// ignore comments inside continuation (https://github.com/moby/moby/issues/29005)
 				continue
 			}
 			line = line[0:len(line)-1] + nextLine
 		}
 
+
+		// TODO *technically* a line like "   RUN     echo  hi    " should be parsed as "RUN" "echo  hi" (cut off instruction, then the rest of the line with TrimSpace), but for our needs "strings.Fields" is good enough for now
+
+		// line = strings.TrimSpace(line) // (emulated below; "strings.Fields" does essentially the same exact thing so we don't need to do it explicitly here too)
+
 		fields := strings.Fields(line)
+
 		if len(fields) < 1 {
-			// must be a much more complex empty line??
+			// ignore empty lines
 			continue
 		}
+
 		instruction := strings.ToUpper(fields[0])
 
 		// TODO balk at ARG / $ in from values

pkg/dockerfile/parse_test.go

@@ -75,6 +75,44 @@ func TestParse(t *testing.T) {
 				Froms: []string{"bash:latest", "busybox:uclibc", "bash:5", "bash:latest", "scratch", "bash:latest", "bash:5", "bash:latest"},
 			},
 		},
+		{
+			name: "empty continuations",
+			dockerfile: `
+				\
+				\
+				\
+				\
+				\
+				\
+			`,
+		},
+		{
+			name: "continuation edge cases",
+			dockerfile: `
+				# continuation does not apply to this comment \
+				FROM scratch
+				# but everything below this is part of a single continuation
+
+				FROM\
+
+				\     
+				\      
+
+				\     
+				\    
+
+				# comments inside are fine
+				# and really yucky empty lines:
+							     						
+				\
+				\
+
+				scratch\
+			`,
+			metadata: dockerfile.Metadata{
+				Froms: []string{"scratch", "scratch"},
+			},
+		},
 		{
 			// TODO is this even something that's supported by classic builder/buildkit? (Tianon *thinks* it was supported once, but maybe he's misremembering and it's never been a thing Dockerfiles, only docker build --target=N ?)
 			name: "numbered stages",