Add pkg/trim-after-delete

This adds a helper which monitors the Docker event stream and, when it
receives an image delete event, queues a call to `fstrim /var/lib/docker`
to trigger a space reclamation. Previously we would rely on a cron job
running every 15 minutes.

Signed-off-by: David Scott <dave.scott@docker.com>

pkg/trim-after-delete/Dockerfile

@@ -0,0 +1,26 @@
+# We need the `fstrim` binary:
+FROM linuxkit/alpine:630ee558e4869672fae230c78364e367b8ea67a9 AS mirror
+RUN mkdir -p /out/etc/apk && cp -r /etc/apk/* /out/etc/apk/
+RUN apk add --no-cache --initdb -p /out \
+    alpine-baselayout \
+    busybox \
+    util-linux
+
+# Remove apk residuals
+RUN rm -rf /out/etc/apk /out/lib/apk /out/var/cache
+
+# We also need the Go binary which calls it:
+RUN apk add --no-cache go musl-dev
+ENV GOPATH=/go PATH=$PATH:/go/bin
+
+COPY .  /go/src/trim-after-delete
+RUN go-compile.sh /go/src/trim-after-delete
+
+FROM scratch
+ENTRYPOINT []
+CMD []
+WORKDIR /
+COPY --from=mirror /out/ /
+COPY --from=mirror /go/bin/trim-after-delete /usr/bin/trim-after-delete
+CMD ["/usr/bin/trim-after-delete", "--", "/sbin/fstrim", "/var/lib/docker"]
+LABEL org.mobyproject.config='{"binds": ["/var/run:/var/run", "/var/lib/docker:/var/lib/docker"], "capabilities": ["CAP_SYS_ADMIN"]}'