This adds a docker remote API proxy to the weaveexec image. The proxy
has hooks for (post-) container start and (pre-) container create; the
former attaches a weave interface to a container given a WEAVE_CIDR
environment entry, and the latter adjusts the entrypoint of a container
to wait for the ethwe interface.
Notes:
None of the hooks need to inspect/change large bodies. In order to avoid
a crash when handling images larger than memory we need to ensure that
the entire request body is never loaded into memory (i.e. No
ioutils.ReadAll).
When proxying a raw-stream response, we can't use http.WriteHeader. It
sets the Transfer-Encoding to chunked, which will throw off the clients.
Instead we have to hijack the socket, and write out http status code
manually.
For chunked transfer-encodings, clients (docker-py) depend on each http
chunk containing exactly one JSON object. Therefore, we cannot re-chunk
the body, and have to pass it across verbatim.
Proxy needs to pass multi-cidrs separately when calling weave. Otherwise
they get smashed together on the command line, and the call fails.
When creating the image, if no entrypoint is specified on the container,
it should use the entrypoint of the image. Code for this is based on
powerstrip-weave.
We handle WEAVE_CIDR='' by acting like it is not set. The reason being,
that because WEAVE_CIDR is inherited from images (along with env vars),
setting it to blank is equivalent to unsetting it.
As noted in #388 comment 76958961, we need the docker client to be as
old as possible while supporting what weave needs to do. At the minute
that means version 1.3.1.
The `apk` package index does not have old versions of docker, so the
most direct way of getting a docker executable is to download the
specific version from https://get.docker.com.
This is to make a visible break with previous releases, such that a
newly downloaded weave script will definitely work.
Without this change in name, an existing weavetools image that doesn't
include the script might be invoked, resulting in a confusing message
for the user.
Michael Bridgen