Install packages without caching

Follows the method used earlier in the file for consistency and means that the apk cache directory doesn't need to be removed.

Dockerfile

@@ -17,12 +17,11 @@ RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -a -tags netgo -o /go/bin/dro
 
 FROM alpine:3.9
 
-RUN apk -U add \
+RUN apk add --no-cache \
     ca-certificates \
     git \
     wget \
-    openssh-client && \
-    rm -rf /var/cache/apk/*
+    openssh-client
 
 ARG terraform_version
 RUN wget -q https://releases.hashicorp.com/terraform/${terraform_version}/terraform_${terraform_version}_linux_amd64.zip -O terraform.zip && \