Wrap weave-kube/launch.sh within tini.

Fixes #2836, i.e. prevents from generating defunct (a.k.a. zombie) launch.sh processes, and also propagates signals from Docker to our processes (i.e. does not reopen #2684).
Why: In Docker, ENTRYPOINT is PID 1 and therefore has the responsibility of reaping processes and forwarding signals to child processes, which launch.sh currently does not do.
This change leverages tini to bake such behaviour in, as recommended by Docker.

See also:
- github.com/docker-library/official-images#init
- github.com/krallin/tini/issues/8

Sample output:

- During initialisation:

```
$ ps auxf
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
[...]
root      1380  0.5  1.9 879660 74864 ?        Ssl  Mar13   5:42 /usr/bin/docker daemon -H fd:// -H unix:///
root      1664  0.0  0.4 502116 18644 ?        Ssl  Mar13   0:05  \_ docker-containerd -l /var/run/docker/li
root      9716  0.0  0.1 134960  5412 ?        Sl   11:00   0:00      \_ docker-containerd-shim 4946a0467c5a
root      9734  0.0  0.0    736     4 ?        Ss   11:00   0:00      |   \_ /sbin/tini -s -- /home/weave/la
root      9738  6.0  1.5 483756 59948 ?        Sl   11:00   0:00      |       \_ /home/weave/weaver --port=6
root     10020  0.0  0.0   1524    64 ?        S    11:00   0:00      |       \_ /bin/sh /home/weave/launch.
root     10110  0.0  0.0   1772  1264 ?        S    11:00   0:00      |           \_ /bin/sh /home/weave/wea
root     10135  0.0  0.0   1772   324 ?        S    11:00   0:00      |               \_ /bin/sh /home/weave
root     10136  0.0  0.0  14656  2912 ?        S    11:00   0:00      |                   \_ curl -o /tmp/we
[...]
```

- Once initialised successfully:

```
$ ps auxf
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
[...]
root      1380  0.5  1.9 879660 74864 ?        Ssl  Mar13   5:42 /usr/bin/docker daemon -H fd:// -H unix:///
root      1664  0.0  0.4 502116 18644 ?        Ssl  Mar13   0:05  \_ docker-containerd -l /var/run/docker/li
root      9716  0.0  0.1 134960  5412 ?        Sl   11:00   0:00      \_ docker-containerd-shim 4946a0467c5a
root      9734  0.0  0.0    736     4 ?        Ss   11:00   0:00      |   \_ /sbin/tini -s -- /home/weave/la
root      9738  4.2  1.5 491952 59948 ?        Sl   11:00   0:00      |       \_ /home/weave/weaver --port=6
[...]
```

prog/weave-kube/Dockerfile.template

@@ -1,3 +1,7 @@
 FROM DOCKERHUB_USER/weaveARCH_EXT
 ADD ./launch.sh ./kube-peers /home/weave/
-ENTRYPOINT ["/home/weave/launch.sh"]
+
+# Add tini for signal processing and zombie killing:
+RUN apk add --update tini
+# Run launch.sh script under tini:
+ENTRYPOINT ["/sbin/tini", "--", "/home/weave/launch.sh"]

prog/weave-kube/launch.sh

@@ -115,8 +115,10 @@ post_start_actions() {
     /home/weave/weave --local expose $WEAVE_EXPOSE_IP
 }
 
-post_start_actions &
+# Double-forks so that post_start_actions' process gets adopted by tini as the subshell () process immediately dies:
+( post_start_actions & )
 
+# Replaces the current shell with weaver. No new process is created.
 exec /home/weave/weaver $EXTRA_ARGS --port=6783 $BRIDGE_OPTIONS \
      --http-addr=$HTTP_ADDR --status-addr=$STATUS_ADDR --docker-api='' --no-dns \
      --ipalloc-range=$IPALLOC_RANGE $NICKNAME_ARG \

prog/weave-kube/weave-daemonset.yaml

@@ -25,8 +25,7 @@ spec:
         - name: weave
           image: weaveworks/weave-kube:latest
           imagePullPolicy: Always
-          command:
-            - /home/weave/launch.sh
+          command: ["/sbin/tini", "-s", "--", "/home/weave/launch.sh"]
           livenessProbe:
             initialDelaySeconds: 30
             httpGet: