Merge pull request #1199 from ethereum-optimism/fix/alpine-image

docker: pin to alpine 3.13 for l2geth

ops/docker/Dockerfile.geth

 # Build Geth in a stock Go builder container
-FROM golang:1.15-alpine as builder
+FROM golang:1.15-alpine3.13 as builder
 
 RUN apk add --no-cache make gcc musl-dev linux-headers git
 
@@ -7,7 +7,7 @@ ADD ./l2geth /go-ethereum
 RUN cd /go-ethereum && make geth
 
 # Pull Geth into a second stage deploy alpine container
-FROM alpine:latest
+FROM alpine:3.13
 
 RUN apk add --no-cache ca-certificates jq curl
 COPY --from=builder /go-ethereum/build/bin/geth /usr/local/bin/