duplicate `task.config.extra_hosts` to Connect sidecar tasks

### Nomad version
Output from `nomad version`
Nomad v1.1.3 (8c0c8140997329136971e66e4c2337dfcf932692)

### Operating system and Environment details
Rocky Linux 8.4 (Green Obsidian)
Docker version 20.10.8, build 3967b7d

```
# nomad agent-info
client
heartbeat_ttl = 12.695519612s
known_servers = 192.168.0.206:4647
last_heartbeat = 2.751359289s
node_id = 3877b77f-6600-7fa3-eaea-6f1dc36d9128
num_allocations = 27
nomad
bootstrap = true
known_regions = 1
leader = true
leader_addr = 192.168.0.206:4647
server = true
raft
applied_index = 21567
commit_index = 21567
fsm_pending = 0
last_contact = 0
last_log_index = 21567
last_log_term = 26
last_snapshot_index = 16628
last_snapshot_term = 23
latest_configuration = [{Suffrage:Voter ID:192.168.0.206:4647 Address:192.168.0.206:4647}]
latest_configuration_index = 0
num_peers = 0
protocol_version = 2
protocol_version_max = 3
protocol_version_min = 0
snapshot_version_max = 1
snapshot_version_min = 0
state = Leader
term = 26
runtime
arch = amd64
cpu_count = 12
goroutines = 2577
kernel.name = linux
max_procs = 12
version = go1.16.5
serf
coordinate_resets = 0
encrypted = false
event_queue = 0
event_time = 1
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 1
members = 1
query_queue = 0
query_time = 1
vault
token_expire_time = 2021-09-15T19:00:39+02:00
token_ttl = 764h46m6s
tracked_for_revoked = 0
```
### Issue
`task.extra_hosts` is not propagated into the docker containers `/etc/hosts`
### Reproduction steps
- Run Consul Connect enabled nomad with a docker driver
- Execute the job file below
- run `cat /etc/hosts` using either `nomad alloc exec` or `docker exec` on the allocation/container

#### Expected Result
```
[root@63ec9326ae5a /]# cat /etc/hosts
# this file was generated by Nomad
127.0.0.1 localhost
::1 localhost
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts

# this entry is the IP address and hostname of the allocation
# shared with tasks in the task group's network
172.26.65.239 63ec9326ae5a

# these entries are extra hosts added by the task config
127.0.0.1 freeipa.ingress.dc1.consul
[root@63ec9326ae5a /]#
```
#### Actual Result
```
[root@63ec9326ae5a /]# cat /etc/hosts
# this file was generated by Nomad
127.0.0.1 localhost
::1 localhost
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts

# this entry is the IP address and hostname of the allocation
# shared with tasks in the task group's network
172.26.65.239 63ec9326ae5a
[root@63ec9326ae5a /]#
```
### Job file (if appropriate)
```hcl
job "freeipa" {
datacenters = ["dc1"]
group "freeipa" {
network {
mode = "bridge"
}
service {
name = "freeipa"
port = "443"
connect {
sidecar_service {}
}
}
task "freeipa" {
resources {
memory = 2000
}
driver = "docker"
config {
image = "freeipa/freeipa-server:centos-8"
args = [ "ipa-server-install", "-U", "-r", "DC1.CONSUL", "--no-ntp" ]
sysctl = {
"net.ipv6.conf.all.disable_ipv6" = "0"
}
extra_hosts = ["freeipa.ingress.dc1.consul:127.0.0.1"]
}
env {
HOSTNAME = "freeipa.ingress.dc1.consul"
PASSWORD = "testtest"
}
}
}
}
```

### Nomad Server logs (if appropriate)

### Nomad Client logs (if appropriate)

See also https://github.com/hashicorp/nomad/issues/7746#issuecomment-898945862

No source file or test is named. Start by tracing Docker task setup for Consul Connect sidecars, using the supplied job and checking /etc/hosts in both the allocation and sidecar containers. Done means task.config.extra_hosts is present in the Connect sidecar container as well.

Written by the indexing model from the issue text.