add build-base to final stage, source stage is unecessary

Dockerfile

@@ -1,25 +1,8 @@
-FROM alpine:latest AS source
+FROM elixir:1.7.4-alpine AS build
 RUN apk add --no-cache --update \
-    bash \
-    ghostscript \
-    imagemagick \
-    libcap \
-    openssl \
-    sudo \
-    tini
-ENV LANG C.UTF-8
-ENV PORT 80
-ENV IMAGER_USER nobody
-HEALTHCHECK --timeout=5s --interval=10s CMD imager ping
-ENTRYPOINT ["/sbin/tini", "--", "/usr/local/bin/imager"]
-CMD ["foreground"]
-
-FROM elixir:1.7.3-alpine AS build
-RUN apk add --update \
     build-base \
     git \
-    libcap-dev \
-    && rm -rf /var/cache/apk/*
+    libcap-dev
 ENV MIX_ENV prod
 ENV OPTIMIZE true
 RUN mkdir /app
@@ -31,6 +14,21 @@ RUN mix do deps.get, deps.compile
 COPY . /app
 RUN mix do compile, release --env=prod
 
-FROM source
+FROM alpine:latest
 MAINTAINER Łukasz Jan Niemier <lukasz.niemier@appunite.com>
+RUN apk add --no-cache --update \
+    build-base \
+    bash \
+    ghostscript \
+    imagemagick \
+    libcap \
+    openssl \
+    sudo \
+    tini
+ENV LANG C.UTF-8
+ENV PORT 80
+ENV IMAGER_USER nobody
 COPY --from=build /app/_build/prod/rel/imager /usr/local
+HEALTHCHECK --timeout=5s --interval=10s CMD imager ping
+ENTRYPOINT ["tini", "--", "/usr/local/bin/imager"]
+CMD ["foreground"]