Switch all Dockerfiles to use "buster" variant

Commit 4e3ab9e9fb switched the
main Dockerfile to the "buster" variant, but did not update
some of the other Dockerfiles.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Dockerfile.buildx

@@ -2,7 +2,7 @@ ARG GO_VERSION=1.13.12
 ARG BUILDX_COMMIT=v0.3.1
 ARG BUILDX_REPO=https://github.com/docker/buildx.git
 
-FROM golang:${GO_VERSION}-stretch AS build
+FROM golang:${GO_VERSION}-buster AS build
 ARG BUILDX_REPO
 RUN git clone "${BUILDX_REPO}" /buildx
 WORKDIR /buildx
@@ -21,6 +21,6 @@ RUN GOOS="${GOOS}" GOARCH="${GOARCH}" BUILDX_COMMIT="${BUILDX_COMMIT}"; \
     "; \
     go build -mod=vendor -ldflags "${ldflags}" -o /usr/bin/buildx ./cmd/buildx
 
-FROM golang:${GO_VERSION}-stretch
+FROM golang:${GO_VERSION}-buster
 COPY --from=build /usr/bin/buildx /usr/bin/buildx
 ENTRYPOINT ["/usr/bin/buildx"]

Dockerfile.simple

@@ -7,7 +7,7 @@
 
 ARG GO_VERSION=1.13.12
 
-FROM golang:${GO_VERSION}-stretch
+FROM golang:${GO_VERSION}-buster
 ENV GO111MODULE=off
 
 # allow replacing httpredir or deb mirror