fix: dockerfile2llb: handle escaping of backslashes correctly

`shlex` in was treating single backslash "\\" in string as
escape sequence, hence stripping them off. Modify `processCmdEnv`
to double escape `\\`.

This was in turn affecting the vertex `Name`, and mostly pronounced
on Windows where backslashes are prevalent in path names. For example
`C:\hello\world\path` was being rendered as `C:helloworldpath`, hence
confusing users.

Fixes #5250

Build progress before fix:
```
	#5 [2/4] RUN echo C:helloworldpath
	#5 1.359 C:\hello\world\path
	#5 DONE 1.7s

	#6 [3/4] RUN echo C:\hello\escaped\path
	#6 1.734 C:\\hello\\escaped\\path
	#6 DONE 2.1s

	#7 [4/4] RUN echo "C:\hello\quoted\path"
	#7 1.765 "C:\hello\quoted\path"
	#7 DONE 2.1s
```

Build progress after fix:
```
	#5 [2/4] RUN echo C:\hello\world\path
	#5 1.458 C:\hello\world\path
	#5 DONE 1.8s

	#6 [3/4] RUN echo C:\\hello\\escaped\\path
	#6 1.730 C:\\hello\\escaped\\path
	#6 DONE 2.1s

	#7 [4/4] RUN echo "C:\hello\quoted\path"
	#7 1.702 "C:\hello\quoted\path"
	#7 DONE 2.1s
```

You can also see that now paths in the step/vertex names now
match those in the log lines right after the name.

Signed-off-by: Anthony Nandaa <[email protected]>

frontend/dockerfile/dockerfile2llb/convert.go

@@ -2052,6 +2052,8 @@ func uppercaseCmd(str string) string {
 }
 
 func processCmdEnv(shlex *shell.Lex, cmd string, env shell.EnvGetter) string {
+	// since shlex is tripping off backslashes
+	cmd = strings.ReplaceAll(cmd, "\\", "\\\\")
 	w, _, err := shlex.ProcessWord(cmd, env)
 	if err != nil {
 		return cmd

frontend/dockerfile/dockerfile2llb/convert_test.go

@@ -230,3 +230,54 @@ RUN echo bar
 	assert.Equal(t, []digest.Digest{"sha256:2e112031b4b923a873c8b3d685d48037e4d5ccd967b658743d93a6e56c3064b9"}, baseImg.RootFS.DiffIDs)
 	assert.Equal(t, "2024-01-17 21:49:12 +0000 UTC", baseImg.Created.String())
 }
+
+// MockEnvGetter is a mock implementation of shell.EnvGetter for testing purposes.
+type MockEnvGetter struct{}
+
+func (m *MockEnvGetter) Get(key string) (string, bool) {
+	return "", false
+}
+
+func (m *MockEnvGetter) Keys() []string {
+	return []string{}
+}
+
+func TestProcessCmdEnv(t *testing.T) {
+	tests := []struct {
+		name     string
+		cmd      string
+		expected string
+	}{
+		{
+			name:     "basic command without backslashes",
+			cmd:      "echo hello world",
+			expected: "echo hello world",
+		},
+		{
+			name:     "command with single backslashes",
+			cmd:      `echo C:\hello\world`,
+			expected: `echo C:\hello\world`,
+		},
+		{
+			name:     "command with multiple backslashes",
+			cmd:      `echo C:\\hello\\world`,
+			expected: `echo C:\\hello\\world`,
+		},
+		{
+			name:     "command with mixed content",
+			cmd:      `echo "C:\hello\world" && dir C:\test\path`,
+			expected: `echo "C:\hello\world" && dir C:\test\path`,
+		},
+	}
+
+	shlex := shell.NewLex('\\')
+	shlex.RawQuotes = true
+	env := &MockEnvGetter{}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := processCmdEnv(shlex, tt.cmd, env)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}