Dockerfile: always use alpine:latest (#5714)

Dockerfile.aarch64

@@ -1,4 +1,4 @@
-FROM aarch64/alpine:3.5
+FROM arm64v8/alpine:latest
 
 # Install system utils & Gogs runtime dependencies
 ADD https://github.com/tianon/gosu/releases/download/1.9/gosu-arm64 /usr/sbin/gosu

Dockerfile.aarch64hub

@@ -1,4 +1,4 @@
-FROM arm64v8/alpine:3.8
+FROM arm64v8/alpine:latest
 
 ENV GOGS_CUSTOM /data/gogs
 ENV QEMU_EXECVE 1

Dockerfile.rpi

@@ -1,4 +1,4 @@
-FROM armhf/alpine:3.5
+FROM arm32v7/alpine:latest
 
 # Install system utils & Gogs runtime dependencies
 ADD https://github.com/tianon/gosu/releases/download/1.9/gosu-armhf /usr/sbin/gosu

Dockerfile.rpihub

@@ -1,4 +1,4 @@
-FROM armhf/alpine:3.5
+FROM arm32v7/alpine:latest
 
 ENV GOGS_CUSTOM /data/gogs
 ENV QEMU_EXECVE 1