use Docker cache mount for apk

dockerfiles/alpine.Dockerfile

@@ -10,11 +10,10 @@ ENV RACK_ENV="production" \
 
 WORKDIR /kubernaut
 
-RUN <<EOT
-  apk update -q
+RUN \
+  --mount=type=cache,id=var-cache-apk,target=/var/cache/apk,sharing=locked \
+  apk update -q; \
   apk add bash jemalloc
-  rm -rf /var/cache/apk
-EOT
 
 RUN \
   --mount=type=cache,id=usr-local-bundle-cache,target=${BUNDLE_PATH},sharing=locked \
@@ -23,10 +22,11 @@ RUN \
 
 FROM base AS build
 
-RUN <<EOT
-  apk add musl-dev gcc make
-  rm -rf /var/cache/apk
-EOT
+RUN \
+  --mount=type=cache,id=var-cache-apk,target=/var/cache/apk,sharing=locked \
+  apk update -q; \
+  apk add musl-dev gcc make; \
+  apk add bash jemalloc
 
 COPY Gemfile Gemfile.lock ./