general tidy up of Dockerfile, make one layer

Dockerfile

@@ -4,46 +4,47 @@ MAINTAINER sparklyballs
 # set version for s6 overlay
 ARG OVERLAY_VERSION="v1.18.1.5"
 ARG OVERLAY_ARCH="amd64"
-ARG OVERLAY_URL="https://github.com/just-containers/s6-overlay/releases/download"
-ARG OVERLAY_WWW="${OVERLAY_URL}"/"${OVERLAY_VERSION}"/s6-overlay-"${OVERLAY_ARCH}".tar.gz
 
-# set some environment variables
+# environment variables
 ENV PS1="$(whoami)@$(hostname):$(pwd)$ " \
 HOME="/root" \
 TERM="xterm"
 
-# add packages
+# install packages
 RUN \
  apk add --no-cache --virtual=build-dependencies \
 	curl \
 	tar && \
-
  apk add --no-cache \
 	bash \
 	tzdata && \
-
-apk add --no-cache --repository http://nl.alpinelinux.org/alpine/edge/community \
+ apk add --no-cache \
+	--repository http://nl.alpinelinux.org/alpine/edge/community \
 	shadow && \
 
 # add s6 overlay
  curl -o \
  /tmp/s6-overlay.tar.gz -L \
-	"${OVERLAY_WWW}" && \
- tar xvfz /tmp/s6-overlay.tar.gz -C / && \
-
-# clean up
- apk del --purge \
-	build-dependencies && \
- rm -rf /var/cache/apk/* /tmp/*
+	"https://github.com/just-containers/s6-overlay/releases/download/${OVERLAY_VERSION}/s6-overlay-${OVERLAY_ARCH}.tar.gz" && \
+ tar xfz \
+	/tmp/s6-overlay.tar.gz -C / && \
 
 # create abc user
-RUN \
  groupmod -g 1000 users && \
  useradd -u 911 -U -d /config -s /bin/false abc && \
  usermod -G users abc && \
 
-# create some folders
-	mkdir -p /config /app /defaults
+# make our folders
+ mkdir -p \
+	/app \
+	/config \
+	/defaults && \
+
+# clean up
+ apk del --purge \
+	build-dependencies && \
+ rm -rf \
+	/tmp/*
 
 # add local files
 COPY root/ /