Bump alpine to 3.10 and other docker improvements.

docker/Dockerfile.amd64

@@ -1,4 +1,4 @@
-FROM alpine:3.9
+FROM alpine:3.10
 
 # Copy files
 ADD . /app

docker/Dockerfile.arm32v6

@@ -1,4 +1,4 @@
-FROM arm32v6/alpine:3.9
+FROM arm32v6/alpine:3.10
 
 # Setup cross-build
 ENV QEMU_EXECVE 1

docker/Dockerfile.arm64v8

@@ -1,4 +1,4 @@
-FROM arm64v8/alpine:3.9
+FROM arm64v8/alpine:3.10
 
 # Setup cross-build
 ENV QEMU_EXECVE 1