Non-root Docker image

[skomma]

Currently, this docker image runs as root.
But described in the official documentation, I think it would be better that this image runs as a non-root user.

Additionally, it is very helpful to install "tini" described in Handling Kernel Signals when running Growi on k8s cluster.

[yuki-takei]

@skomma

Currently, this docker image runs as root.
But described in the official documentation, I think it would be better that this image runs as a non-root user.

We'll fix it.

Additionally, it is very helpful to install "tini" described in Handling Kernel Signals when running Growi on k8s cluster.

I think this image is not needed to use tini because this uses jwilder/dockerize and it seems to support SIGTERM.
(see jwilder/dockerize#12 (comment))