Dockerfile: add GOTOOLCHAIN

Changes in go1.21 means the build recipe here was doing the wrong thing.
This should restore the expected behavior of using the toolchain present
in the container.

Signed-off-by: Hank Donnay <hdonnay@redhat.com>

Dockerfile

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+ARG GOTOOLCHAIN=local
 ARG GO_VERSION=1.21
 FROM quay.io/projectquay/golang:${GO_VERSION} AS build
 WORKDIR /build