build: using targetarch for arm support

Dockerfile

@@ -1,6 +1,7 @@
 # Build the manager binary
-FROM quay.io/bitnami/golang:1.16 as builder
+FROM golang:1.16 as builder
 
+ARG TARGETARCH
 ARG GIT_HEAD_COMMIT
 ARG GIT_TAG_COMMIT
 ARG GIT_LAST_TAG
@@ -24,7 +25,7 @@ COPY controllers/ controllers/
 COPY pkg/ pkg/
 
 # Build
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 GO111MODULE=on go build \
+RUN CGO_ENABLED=0 GOOS=linux GOARCH=$TARGETARCH GO111MODULE=on go build \
         -gcflags "-N -l" \
         -ldflags "-X main.GitRepo=$GIT_REPO -X main.GitTag=$GIT_LAST_TAG -X main.GitCommit=$GIT_HEAD_COMMIT -X main.GitDirty=$GIT_MODIFIED -X main.BuildTime=$BUILD_DATE" \
         -o manager