Dockerfile: Go back to scratch

Now that we can build with CGO_ENABLED=0 we do not have to use a base
image that provides libc. Remove distroless for the normal and
rootless images and use scratch instead.

Signed-off-by: Torin Sandall <torinsandall@gmail.com>

Dockerfile

@@ -2,9 +2,9 @@
 # Use of this source code is governed by an Apache2
 # license that can be found in the LICENSE file.
 
-ARG VARIANT
+ARG BASE
 
-FROM gcr.io/distroless/base${VARIANT}
+FROM ${BASE}
 
 # Any non-zero number will do, and unfortunately a named user will not, as k8s
 # pod securityContext runAsNonRoot can't resolve the user ID:

Makefile

@@ -212,9 +212,9 @@ build-windows:
 
 .PHONY: image-quick
 image-quick:
-	$(DOCKER) build -t $(IMAGE):$(VERSION) .
-	$(DOCKER) build -t $(IMAGE):$(VERSION)-debug --build-arg VARIANT=:debug .
-	$(DOCKER) build -t $(IMAGE):$(VERSION)-rootless --build-arg USER=1000 .
+	$(DOCKER) build -t $(IMAGE):$(VERSION) --build-arg BASE=scratch .
+	$(DOCKER) build -t $(IMAGE):$(VERSION)-debug --build-arg BASE=gcr.io/distroless/base:debug .
+	$(DOCKER) build -t $(IMAGE):$(VERSION)-rootless --build-arg USER=1000 --build-arg BASE=scratch .
 
 .PHONY: push
 push: