Fix environment-only Dockerfile builds (#47)

* Fix environment-only Dockerfile builds

* extract marker path to a constant

Author: realrajaryan

pkg/build/frontend.go

@@ -378,9 +378,37 @@ func solvePlatform(ctx context.Context, bopts *BOpts, pl ocispecs.Platform, c ga
 		return nil, nil, err
 	}
 
-	// This only happens when the dockerfile is just `FROM scratch`
+	// Handle metadata-only builds (ENV/ARG/LABEL without RUN/COPY/ADD)
 	if ref == nil {
-		return nil, nil, ErrNoBuildDirectives
+		if img == nil {
+			return nil, nil, ErrNoBuildDirectives
+		}
+
+		// OCI manifests require a layers array (cannot be null)
+		const metadataOnlyMarker = "/.container-metadata-only"
+		stateWithLayer := llb.Scratch().
+			File(llb.Mkfile(metadataOnlyMarker, 0644, []byte("# This image contains only metadata (ENV/ARG/LABEL)\n"))).
+			Platform(pl)
+
+		layerDef, err := stateWithLayer.Marshal(ctx)
+		if err != nil {
+			return nil, nil, err
+		}
+
+		layerResult, err := c.Solve(ctx, gateway.SolveRequest{
+			Evaluate:       false,
+			Definition:     layerDef.ToPB(),
+			FrontendOpt:    frontendOpt,
+			FrontendInputs: frontendInputs,
+		})
+		if err != nil {
+			return nil, nil, err
+		}
+
+		ref, err = layerResult.SingleRef()
+		if err != nil {
+			return nil, nil, err
+		}
 	}
 
 	_, err = ref.ToState()

pkg/build/frontend_test.go

@@ -621,6 +621,93 @@ COPY --from=builder /app /app`,
 			expectedStates: 2,
 			wantErr:        false,
 		},
+		// Environment-only Dockerfile tests
+		{
+			name: "FROM scratch with only ENV directives",
+			dockerfile: `FROM scratch
+
+ARG BUILD_DATE
+ENV TERM=xterm \
+    BUILD_DATE=${BUILD_DATE}`,
+			buildPlatforms:        []ocispecs.Platform{{OS: "linux", Architecture: "arm64"}},
+			targetPlatform:        ocispecs.Platform{OS: "linux", Architecture: "arm64"},
+			buildArgs:             map[string]string{"BUILD_DATE": "2025-01-01"},
+			target:                "",
+			expectedResolverCalls: []testResolverCall{}, // No resolver calls for scratch
+			expectedStates:        0,
+			wantErr:               false,
+			errContains:           "",
+		},
+		{
+			name: "FROM alpine with only ENV and LABEL",
+			dockerfile: `FROM alpine:latest
+ENV APP_VERSION=1.0.0 \
+    APP_NAME=myapp
+LABEL maintainer="[email protected]" \
+      version="1.0.0"`,
+			buildPlatforms: []ocispecs.Platform{{OS: "linux", Architecture: "arm64"}},
+			targetPlatform: ocispecs.Platform{OS: "linux", Architecture: "arm64"},
+			buildArgs:      map[string]string{},
+			target:         "",
+			expectedResolverCalls: []testResolverCall{
+				{ref: "alpine:latest", platform: &ocispecs.Platform{OS: "linux", Architecture: "arm64"}},
+			},
+			expectedStates: 1,
+			wantErr:        false,
+			errContains:    "",
+		},
+		{
+			name: "Complex ARG and ENV combinations",
+			dockerfile: `FROM scratch
+ARG JOBS=6
+ARG ARCH=amd64
+ENV MAKEOPTS="-j${JOBS}" \
+    ARCH="${ARCH}" \
+    PATH="/usr/local/bin:/usr/bin"`,
+			buildPlatforms:        []ocispecs.Platform{{OS: "linux", Architecture: "arm64"}},
+			targetPlatform:        ocispecs.Platform{OS: "linux", Architecture: "arm64"},
+			buildArgs:             map[string]string{"JOBS": "8", "ARCH": "arm64"},
+			target:                "",
+			expectedResolverCalls: []testResolverCall{},
+			expectedStates:        0,
+			wantErr:               false,
+			errContains:           "",
+		},
+		{
+			name: "FROM scratch with only LABEL directives",
+			dockerfile: `FROM scratch
+LABEL maintainer="[email protected]" \
+      version="1.0.0" \
+      description="Test image"`,
+			buildPlatforms:        []ocispecs.Platform{{OS: "linux", Architecture: "arm64"}},
+			targetPlatform:        ocispecs.Platform{OS: "linux", Architecture: "arm64"},
+			buildArgs:             map[string]string{},
+			target:                "",
+			expectedResolverCalls: []testResolverCall{},
+			expectedStates:        0,
+			wantErr:               false,
+			errContains:           "",
+		},
+		{
+			name: "FROM debian with ENV ARG and LABEL mix",
+			dockerfile: `FROM debian:bullseye
+ARG VERSION=1.0.0
+ARG [email protected]
+ENV APP_VERSION=${VERSION} \
+    PATH=/app/bin:$PATH
+LABEL maintainer="${MAINTAINER}" \
+      version="${VERSION}"`,
+			buildPlatforms: []ocispecs.Platform{{OS: "linux", Architecture: "amd64"}},
+			targetPlatform: ocispecs.Platform{OS: "linux", Architecture: "amd64"},
+			buildArgs:      map[string]string{"VERSION": "2.0.0", "MAINTAINER": "[email protected]"},
+			target:         "",
+			expectedResolverCalls: []testResolverCall{
+				{ref: "debian:bullseye", platform: &ocispecs.Platform{OS: "linux", Architecture: "amd64"}},
+			},
+			expectedStates: 1,
+			wantErr:        false,
+			errContains:    "",
+		},
 	}
 
 	for _, tt := range tests {