docker: speed up docker image build (#24796)

This PR improves the docker build speed for repeated builds where go.mod and go.sum do no change, by placing the downloaded dependencies in a lower layer

Dockerfile

@@ -8,6 +8,11 @@ FROM golang:1.18-alpine as builder
 
 RUN apk add --no-cache gcc musl-dev linux-headers git
 
+# Get dependencies - will also be cached if we won't change go.mod/go.sum
+COPY go.mod /go-ethereum/
+COPY go.sum /go-ethereum/
+RUN cd /go-ethereum && go mod download
+
 ADD . /go-ethereum
 RUN cd /go-ethereum && go run build/ci.go install ./cmd/geth
 

Dockerfile.alltools

@@ -8,6 +8,11 @@ FROM golang:1.18-alpine as builder
 
 RUN apk add --no-cache gcc musl-dev linux-headers git
 
+# Get dependencies - will also be cached if we won't change go.mod/go.sum
+COPY go.mod /go-ethereum/
+COPY go.sum /go-ethereum/
+RUN cd /go-ethereum && go mod download
+
 ADD . /go-ethereum
 RUN cd /go-ethereum && go run build/ci.go install