Refactor Dockerfile

1. Use `apk -U --no-cache add` instead of `apk update` + `apk add` +
manually cache clean up.
2. Separate package installation and user, group setup, the combination
to reduce a docker image layer didn't bring benefits here, only makes
Dockerfiles more complex.

Dockerfile

@@ -3,8 +3,7 @@ MAINTAINER Thomas Boerger
 
 EXPOSE 22 3000
 
-RUN apk update && \
-  apk add \
+RUN apk -U --no-cache add \
     su-exec \
     ca-certificates \
     sqlite \
@@ -14,10 +13,8 @@ RUN apk update && \
     s6 \
     curl \
     openssh \
-    tzdata && \
-  rm -rf \
-    /var/cache/apk/* && \
-  addgroup \
+    tzdata
+RUN addgroup \
     -S -g 1000 \
     git && \
   adduser \

Dockerfile.aarch64

@@ -2,8 +2,7 @@ FROM aarch64/alpine:3.5
 
 EXPOSE 22 3000
 
-RUN apk update && \
-  apk add \
+RUN apk -U --no-cache add \
     su-exec \
     ca-certificates \
     sqlite \
@@ -13,10 +12,8 @@ RUN apk update && \
     s6 \
     curl \
     openssh \
-    tzdata && \
-  rm -rf \
-    /var/cache/apk/* && \
-  addgroup \
+    tzdata
+RUN addgroup \
     -S -g 1000 \
     git && \
   adduser \

Dockerfile.rpi

@@ -3,8 +3,7 @@ MAINTAINER Thomas Boerger
 
 EXPOSE 22 3000
 
-RUN apk update && \
-  apk add \
+RUN apk -U --no-cache add \
     su-exec \
     ca-certificates \
     sqlite \
@@ -14,10 +13,8 @@ RUN apk update && \
     s6 \
     curl \
     openssh \
-    tzdata && \
-  rm -rf \
-    /var/cache/apk/* && \
-  addgroup \
+    tzdata
+RUN addgroup
     -S -g 1000 \
     git && \
   adduser \