Nomad only pull images supported by defined helper

### Nomad version
Output from `nomad version`
Nomad v0.12.7 (6147cb578794cb2d0c35d68fe1791728a09bb081)
### Operating system and Environment details
> cat /etc/os-release
NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"
PRETTY_NAME="CentOS Linux 7 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:7"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"

### Issue
I have my Nomad running on an on-prem machine and using ECR as a container registry. For a long time, I was setting up credentials helper so it would pull the images from the ECR. Once that was done, everything worked for a while. Until I realized, that once the image of envoyproxy/envoy was gone, sidecar for any service wouldn't start with an error: `Failed to find docker auth for repo "envoyproxy/envoy": docker-credential-ecr-login with input "envoyproxy/envoy" failed with stderr: exit status 1`. Clearly, for some reason, it tries to authenticate against docker.io with ecr-login cred helper.

### Reproduction steps

nomad.hcl
```
data_dir = "/opt/nomad/data"
bind_addr = "0.0.0.0"

server {
enabled = true
bootstrap_expect = 1
}

client {
enabled = true
servers = ["127.0.0.1:4646"]
host_volume "postgres" {
path = "/opt/postgres/data"
read_only = false
}
options = {
"docker.auth.config" = "/etc/docker/config.json"
"docker.auth.helper" = "ecr-login"
}
}

consul {
address = "127.0.0.1:8500"
server_service_name = "nomad"
client_service_name = "nomad-client"
auto_advertise = true
server_auto_join = true
client_auto_join = true
}
```

config.json
```
{
"credHelpers": {
"059899322608.dkr.ecr.us-east-1.amazonaws.com": "ecr-login"
},
"credsStore": "ecr-login"
}
```

### Job file (if appropriate)
```
job "my_job" {

datacenters = ["dc1"]
type = "service"

update {
max_parallel = 1
min_healthy_time = "10s"
healthy_deadline = "3m"
progress_deadline = "10m"
auto_revert = false
canary = 0
}

migrate {
max_parallel = 1
health_check = "checks"
min_healthy_time = "10s"
healthy_deadline = "5m"
}

group "postgres" {

network {
mode = "bridge"
}

volume "postgres" {
type = "host"
read_only = false
source = "postgres"
}

restart {
attempts = 3
interval = "15m"
delay = "15s"
mode = "delay"
}

reschedule {
attempts = 15
interval = "1h"
delay = "30s"
delay_function = "exponential"
max_delay = "120s"
unlimited = false
}

service {
name = "postgres"
tags = ["infra", "db"]
port = 5432

connect {
sidecar_service {}
}

check {
type = "script"
command = "pg_isready"
interval = "60s"
timeout = "5s"
task = "postgres"

check_restart {
limit = 3
grace = "60s"
ignore_warnings = false
}
}
}

task "postgres" {

driver = "docker"

volume_mount {
volume = "postgres"
destination = "/var/lib/postgresql/data"
read_only = false
}

config {
image = "postgres:12"
}

env {
POSTGRES_DB = ""
POSTGRES_USER = ""
POSTGRES_PASSWORD = ""
}

resources {
cpu = 500
memory = 1024
network {
mbits = 10
}
}
}
}

group "my_task_group" {

network {
mode = "bridge"
port "my_service_healthcheck" {
to = -1
}
}

restart {
attempts = 18
interval = "5m"
delay = "15s"
mode = "delay"
}

reschedule {
attempts = 15
interval = "1h"
delay = "30s"
delay_function = "exponential"
max_delay = "120s"
unlimited = false
}

service {
name = "my_service"
tags = ["microservice"]
port = 8030

check {
name = "my-service-health"
type = "http"
port = "my_service_healthcheck"
path = "/v1/health"
interval = "10s"
timeout = "5s"

check_restart {
limit = 3
grace = "60s"
ignore_warnings = false
}
}

connect {
sidecar_service {
proxy {
expose {
path {
path = "/v1/health"
protocol = "http"
local_path_port = 8030
listener_port = "my_service_healthcheck"
}
}
upstreams = {
destination_name = "postgres"
local_bind_port = 5432
}
}
}
}
}

task "my_task" {

driver = "docker"

config {
image = "____ACCOUNT_ID____.dkr.ecr.____REGION____.amazonaws.com/image-name:tag"
force_pull = true
}

env {
DS_DB_NAME = ""
DS_DB_USER = ""
DS_DB_PASSWORD = ""
}

resources {
cpu = 500
memory = 512
network {
mbits = 10
}
}
}
}
}
```
### Nomad Client/Server logs (if appropriate)
Nothing that would be related to this issue...