Hardcode path to docker images (go-gitea#23955)

Fix go-gitea#23954

This allows for building on platforms that don't have docker hub as the
default container registry.

Dockerfile

@@ -1,5 +1,5 @@
 #Build stage
-FROM golang:1.20-alpine3.17 AS build-env
+FROM docker.io/library/golang:1.20-alpine3.17 AS build-env
 
 ARG GOPROXY
 ENV GOPROXY ${GOPROXY:-direct}
@@ -23,7 +23,7 @@ RUN if [ -n "${GITEA_VERSION}" ]; then git checkout "${GITEA_VERSION}"; fi \
 # Begin env-to-ini build
 RUN go build contrib/environment-to-ini/environment-to-ini.go
 
-FROM alpine:3.17
+FROM docker.io/library/alpine:3.17
 LABEL maintainer="maintainers@gitea.io"
 
 EXPOSE 22 3000

Dockerfile.rootless

@@ -1,5 +1,5 @@
 #Build stage
-FROM golang:1.20-alpine3.17 AS build-env
+FROM docker.io/library/golang:1.20-alpine3.17 AS build-env
 
 ARG GOPROXY
 ENV GOPROXY ${GOPROXY:-direct}
@@ -23,7 +23,7 @@ RUN if [ -n "${GITEA_VERSION}" ]; then git checkout "${GITEA_VERSION}"; fi \
 # Begin env-to-ini build
 RUN go build contrib/environment-to-ini/environment-to-ini.go
 
-FROM alpine:3.17
+FROM docker.io/library/alpine:3.17
 LABEL maintainer="maintainers@gitea.io"
 
 EXPOSE 2222 3000