Add pipeline to publish ghcr multi-arch image (#2268)

Refactor operator image build process
Add a pipeline to build and publish arm64/amd64 image in ghcr on every
pushed tag

docker/build_operator.sh

@@ -0,0 +1,31 @@
+#!/bin/bash
+
+export DEBIAN_FRONTEND=noninteractive
+
+arch=$(dpkg --print-architecture)
+
+set -ex
+
+# Install dependencies
+
+apt-get update
+apt-get install -y wget
+
+(
+    cd /tmp
+    wget -q "https://storage.googleapis.com/golang/go1.18.9.linux-${arch}.tar.gz" -O go.tar.gz
+    tar -xf go.tar.gz
+    mv go /usr/local
+    ln -s /usr/local/go/bin/go /usr/bin/go
+    go version
+)
+
+# Build
+
+export PATH="$PATH:$HOME/go/bin"
+export GOPATH="$HOME/go"
+mkdir -p build
+cp scm-source.json build/
+
+GO111MODULE=on go mod vendor
+CGO_ENABLED=0 go build -o build/postgres-operator -v -ldflags "$OPERATOR_LDFLAGS" cmd/main.go