added "light" docker image

[atheiman]

This new docker image is suitable for use as a sidecar container in a Kubernetes pod with the PodShareProcessNamespace feature. There is more information on why the current images are not a good fit in the linked issue.

Closes #1190

[hashicorp-cla]

All committers have signed the CLA.

[atheiman]

@pearkes @kyhavlov you two seem to be the most active contributors to this - could you take a few minutes to review this contribution?

[pearkes]

@atheiman thanks for the contribution and the idea! I don't see any concern with adding this additional target, given it has no dependencies and doesn't add any management overheard I can think of. We'll be doing a Docker release soon (#1192) and this should be available following that.

[atheiman]

If anyone finds this, the light tag has been released: https://hub.docker.com/r/hashicorp/consul-template/tags

$ docker run --rm -it hashicorp/consul-template:light -version
consul-template v0.20.0 (b709612c)
$ docker run --rm -it --entrypoint=/bin/sh hashicorp/consul-template:light
/ # which consul-template
/bin/consul-template
/ # cat /etc/os-release
NAME="Alpine Linux"
ID=alpine
VERSION_ID=3.9.0
PRETTY_NAME="Alpine Linux v3.9"
HOME_URL="https://alpinelinux.org/"
BUG_REPORT_URL="https://bugs.alpinelinux.org/"