tests: add testDockerfileFromHTTP for WCOW

Enables `testDockerfileFromHTTP` integration test
for Windows.

Also fixes a bug where paths were being joined wrongly,
for example:

```go
path.Join("/", "\\context") // would be "/\\context"
// instead use
src := filepath.Join("/", "\\context") // "\context"
```

Addressing tests marked as Revisit for #4485

Signed-off-by: Anthony Nandaa <profnandaa@gmail.com>

client/llb/fileop.go

@@ -5,6 +5,7 @@ import (
 	_ "crypto/sha256" // for opencontainers/go-digest
 	"os"
 	"path"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"time"
@@ -576,7 +577,7 @@ func (a *fileActionCopy) sourcePath(ctx context.Context) (string, error) {
 			return "", err
 		}
 	}
-	return path.Join(dir, p), nil
+	return filepath.Join(dir, p), nil
 }
 
 func (a *fileActionCopy) addCaps(f *FileOp) {

frontend/dockerfile/dockerfile_test.go

@@ -4491,7 +4491,6 @@ COPY --from=build foo bar2
 }
 
 func testDockerfileFromHTTP(t *testing.T, sb integration.Sandbox) {
-	integration.SkipOnPlatform(t, "windows")
 	f := getFrontend(t, sb)
 
 	buf := bytes.NewBuffer(nil)
@@ -4509,9 +4508,10 @@ func testDockerfileFromHTTP(t *testing.T, sb integration.Sandbox) {
 		require.NoError(t, err)
 	}
 
-	writeFile("mydockerfile", `FROM scratch
+	dockerfile := fmt.Sprintf(`FROM %s
 COPY foo bar
-`)
+`, integration.UnixOrWindows("scratch", "nanoserver"))
+	writeFile("mydockerfile", dockerfile)
 
 	writeFile("foo", "foo-contents")