build: normalize policy env filename default

Use cleaned DockerfilePath basename for policy env filename and fall back
to Dockerfile when the path resolves to dot or root.

Previously filename could be reported as "." on default cases.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

build/opt.go

@@ -7,7 +7,6 @@ import (
 	"io"
 	"maps"
 	"os"
-	"path"
 	"path/filepath"
 	"slices"
 	"strconv"
@@ -603,7 +602,7 @@ func configureSourcePolicy(ctx context.Context, np *noderesolver.ResolvedNode, o
 		}
 		env.Args[k] = &v
 	}
-	env.Filename = path.Base(opt.Inputs.DockerfilePath)
+	env.Filename = policyEnvFilename(opt.Inputs)
 	env.Target = opt.Target
 	env.Labels = opt.Labels
 
@@ -695,6 +694,14 @@ func configureSourcePolicy(ctx context.Context, np *noderesolver.ResolvedNode, o
 	return defers, nil
 }
 
+func policyEnvFilename(inp Inputs) string {
+	base := filepath.Base(filepath.Clean(inp.DockerfilePath))
+	if base != "." && base != string(filepath.Separator) {
+		return base
+	}
+	return "Dockerfile"
+}
+
 func loadInputs(ctx context.Context, d *driver.DriverHandle, inp *Inputs, pw progress.Writer, target *client.SolveOpt) (func(), error) {
 	if inp.ContextPath == "" {
 		return nil, errors.New("please specify build context (e.g. \".\" for the current directory)")