Rename .buildx Dockerfiles to .buildkit

This is a more accurate name, since these Dockerfiles require BuildKit, not Buildx.

hooks/build

@@ -34,9 +34,9 @@ for label in "${LABELS[@]}"; do
     LABEL_ARGS+=(--label "${label}")
 done
 
-# Check if DOCKER_BUILDKIT is set, if so, use the Dockerfile.buildx as template
+# Check if DOCKER_BUILDKIT is set, if so, use the Dockerfile.buildkit as template
 if [[ -n "${DOCKER_BUILDKIT}" ]]; then
-    buildx_suffix=.buildx
+    buildkit_suffix=.buildkit
 fi
 
 set -ex
@@ -45,6 +45,6 @@ for arch in "${arches[@]}"; do
     docker build \
            "${LABEL_ARGS[@]}" \
            -t "${DOCKER_REPO}:${DOCKER_TAG}-${arch}" \
-           -f docker/${arch}/Dockerfile${buildx_suffix}${distro_suffix} \
+           -f docker/${arch}/Dockerfile${buildkit_suffix}${distro_suffix} \
            .
 done