Add pkg/host-timesync-daemon

Some hypervisors (e.g. hyperkit / xhyve) don't provide a good way to keep
the VM's clock in sync with the Host's clock. NTP will usually keep the
clocks together, but after a the host or VM is suspended and resumed the
clocks can be suddenly too far apart for NTP to work properly.

This simple daemon listens on an AF_VSOCK port and resynchronises the VM
clock from the virtualised hardware clock.

This is a Go conversion of original C code written by
Magnus Skjegstad <magnus@skjegstad.com>

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

pkg/host-timesync-daemon/Dockerfile

@@ -0,0 +1,22 @@
+FROM linuxkit/alpine:2e362f4459ba4491655061cccdd2fcc7a2de5eb3 AS mirror
+
+RUN apk add --no-cache go musl-dev git
+ENV GOPATH=/go PATH=$PATH:/go/bin
+
+ENV VIRTSOCK_COMMIT=a381dcc5bcddf1d7f449495c373dbf70f8e501c0
+RUN mkdir -p $GOPATH/src/github.com/linuxkit && \
+  cd $GOPATH/src/github.com/linuxkit && \
+  git clone https://github.com/linuxkit/virtsock.git && \
+  cd virtsock && \
+  git checkout $VIRTSOCK_COMMIT
+
+COPY .  /go/src/host-timesync-daemon
+RUN go-compile.sh /go/src/host-timesync-daemon
+
+FROM scratch
+ENTRYPOINT []
+CMD []
+WORKDIR /
+COPY --from=mirror /go/bin/host-timesync-daemon /usr/bin/host-timesync-daemon
+CMD ["/usr/bin/host-timesync-daemon", "-port", "0xf3a4"]
+LABEL org.mobyproject.config='{"binds": [ "/dev/rtc0:/dev/rtc0" ], "capabilities": ["CAP_SYS_TIME"]}'