docs: fix links to connect

driver/docker: ensure that defaults are populated for dangling containers config

Added version requirement to new feature guides

docs: Added alert to mainpage for new 0.10

Merge 0.10.0 release branch back into master

docs: remove link to 0.10.0-rc1

Co-Authored-By: Michael Schurter <mschurter@hashicorp.com>

docs: bump to Nomad 0.10.0

This reverts commit 482862e6.

Fixes #6375

Intentionally omitted IPs prior to 0.10.0 release to minimize changes
and risk.

client: expose group network ports in env vars

Fixes #6375

Intentionally omitted IPs prior to 0.10.0 release to minimize changes
and risk.

Update link to new blog

Looks like we may need to pass default literal at each layer to be able,
so defaults are set properly.

docker: periodically reconcile containers

Other labels aren't strictly necessary here, and we may follow up with a
better way to customize.

Use golang 1.12.12

driver.SetConfig is not appropriate for starting up reconciler
goroutine.  Some ephemeral driver instances are created for validating
config and we ought not to side-effecting goroutines for those.

We currently lack a lifecycle hook to inject these, so I picked the
`Fingerprinter` function for now, and reconciler should only run after
fingerprinter started.

Use `sync.Once` to ensure that we only start reconciler loop once.

Append the Docker image sha to the Envoy image to ensure users default
to using the version that Nomad was tested against.

connect: upgrade to envoy 1.11.2 and add sha

Use golang 1.12.12

Append the Docker image sha to the Envoy image to ensure users default
to using the version that Nomad was tested against.

Use use base official golang image in CircleCI, as it gets refreshed
more quickly compared to circleci/golang, and we don't benefit from
circleci image customizations much.