Realy don't cache apk index

Actualy the "-U --no-cache" effectivly is treated as --no-cache
was never specified:

/ # apk -U --no-cache add sed
fetch http://dl-cdn.alpinelinux.org/alpine/v3.5/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.5/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.5/community/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.5/community/x86_64/APKINDEX.tar.gz
(1/1) Installing sed (4.2.2-r0)
Executing busybox-1.25.1-r0.trigger
OK: 4 MiB in 12 packages
/ # ls -la /var/cache/apk/
total 1004
drwxr-xr-x    1 root     root            70 May  8 11:08 .
drwxr-xr-x    1 root     root            29 Mar  3 11:20 ..
-rw-r--r--    1 root     root        732807 May  5 07:11 APKINDEX.c51f8f92.tar.gz
-rw-r--r--    1 root     root        293014 May  1 12:37 APKINDEX.d09172fd.tar.gz

So removing the "-U" fix this behavior.

Dockerfile

@@ -3,7 +3,7 @@ MAINTAINER Thomas Boerger <thomas@webhippie.de>
 
 EXPOSE 22 3000
 
-RUN apk -U --no-cache add \
+RUN apk --no-cache add \
     su-exec \
     ca-certificates \
     sqlite \

Dockerfile.aarch64

@@ -2,7 +2,7 @@ FROM multiarch/alpine:aarch64-v3.5
 
 EXPOSE 22 3000
 
-RUN apk -U --no-cache add \
+RUN apk --no-cache add \
     su-exec \
     ca-certificates \
     sqlite \

Dockerfile.rpi

@@ -3,7 +3,7 @@ MAINTAINER Thomas Boerger <thomas@webhippie.de>
 
 EXPOSE 22 3000
 
-RUN apk -U --no-cache add \
+RUN apk --no-cache add \
     su-exec \
     ca-certificates \
     sqlite \