update docker build commands in Makefiles to use buildx (#3137)

* switch to docker buildx and update docs
* minor update to e2e Dockerfile

docker/Dockerfile

@@ -8,12 +8,11 @@ ARG TARGETARCH
 COPY  . /go/src/github.com/zalando/postgres-operator
 WORKDIR /go/src/github.com/zalando/postgres-operator
 
-RUN go clean -cache -modcache
 RUN go mod vendor \
     && CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -o build/postgres-operator -v -ldflags "-X=main.version=${VERSION}" cmd/main.go
 
 ARG BASE_IMAGE
-FROM --platform=$TARGETPLATFORM ${BASE_IMAGE}
+FROM ${BASE_IMAGE}
 LABEL maintainer="Team ACID @ Zalando <team-acid@zalando.de>"
 LABEL org.opencontainers.image.source="https://github.com/zalando/postgres-operator"
 

docker/build_operator.sh

@@ -26,6 +26,5 @@ export PATH="$PATH:$HOME/go/bin"
 export GOPATH="$HOME/go"
 mkdir -p build
 
-go clean -cache -modcache
 go mod vendor
 CGO_ENABLED=0 go build -o build/postgres-operator -v -ldflags "$OPERATOR_LDFLAGS" cmd/main.go