Make fails cryptically if docker-squash is not installed

[peterbourgon]

Removing intermediate container 0786e37577cb
Successfully built bfaf16cdf9ad
docker save weaveworks/scope:latest |   -t weaveworks/scope | tee scope.tar |  docker load
/bin/sh: 1: -t: not found
FATA[0000] Error response from daemon: EOF

Makefile should check that all dependencies exist, and produce actionable error messages if something needs to be installed.

[peterbourgon]

Also, docker-squash apparently cannot be run without sudo, even if your Docker is set up to not require it. So probably also need this patch:

diff --git a/Makefile b/Makefile
index 40700fe..f59b9a3 100644
--- a/Makefile
+++ b/Makefile
@@ -18,7 +18,7 @@ all: $(SCOPE_EXPORT)
 $(SCOPE_EXPORT): $(APP_EXE) $(PROBE_EXE) docker/*
        cp $(APP_EXE) $(PROBE_EXE) docker/
        $(SUDO) docker build -t $(SCOPE_IMAGE) docker/
-       $(SUDO) docker save $(SCOPE_IMAGE):latest | $(SUDO) $(DOCKER_SQUASH) -t $(SCOPE_IMAGE) | tee $@ | $(SUDO) docker load
+       $(SUDO) docker save $(SCOPE_IMAGE):latest | sudo $(DOCKER_SQUASH) -t $(SCOPE_IMAGE) | tee $@ | $(SUDO) docker load

 $(APP_EXE): app/*.go report/*.go xfer/*.go