chore: add LB_DNS env var for internal ALBs (#5483)

```
Variables

  Name                                Container  Environment           Value
  ----                                ---------  -----------           -----
  COPILOT_APPLICATION_NAME            be         env-name-with-dashes  v129-testing
    "                                   "        test                    "
  COPILOT_ENVIRONMENT_NAME              "        env-name-with-dashes  env-name-with-dashes
    "                                   "        test                  test
  COPILOT_LB_DNS                        "        env-name-with-dashes  internal-internalALBforImport-1595089742.us-west-2.elb.amazonaws.com
  COPILOT_SERVICE_DISCOVERY_ENDPOINT    "          "                   env-name-with-dashes.v129-testing.local
    "                                   "        test                  test.v129-testing.local
  COPILOT_SERVICE_NAME                  "        env-name-with-dashes  be
    "                                   "        test                    "
```

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internal/pkg/deploy/cloudformation/stack/testdata/workloads/backend/http-autoscaling-template.yml

@@ -85,6 +85,8 @@ Resources:
               Value: !Sub "${EnvName}"
             - Name: COPILOT_SERVICE_NAME
               Value: !Sub "${WorkloadName}"
+            - Name: COPILOT_LB_DNS
+              Value: !GetAtt EnvControllerAction.InternalLoadBalancerDNSName
           EnvironmentFiles:
             - !If
               - HasEnvFile

internal/pkg/deploy/cloudformation/stack/testdata/workloads/backend/http-full-config-template.yml

@@ -90,6 +90,8 @@ Resources:
               Value: !Sub "${EnvName}"
             - Name: COPILOT_SERVICE_NAME
               Value: !Sub "${WorkloadName}"
+            - Name: COPILOT_LB_DNS
+              Value: !GetAtt EnvControllerAction.InternalLoadBalancerDNSName
           EnvironmentFiles:
             - !If
               - HasEnvFile
@@ -119,6 +121,8 @@ Resources:
               Value: !Sub "${EnvName}"
             - Name: COPILOT_SERVICE_NAME
               Value: !Sub "${WorkloadName}"
+            - Name: COPILOT_LB_DNS
+              Value: !GetAtt EnvControllerAction.InternalLoadBalancerDNSName
           EnvironmentFiles:
             - !If
               - HasEnvFileForfluentbit

internal/pkg/deploy/cloudformation/stack/testdata/workloads/backend/http-only-path-template.yml

@@ -85,6 +85,8 @@ Resources:
               Value: !Sub "${EnvName}"
             - Name: COPILOT_SERVICE_NAME
               Value: !Sub "${WorkloadName}"
+            - Name: COPILOT_LB_DNS
+              Value: !GetAtt EnvControllerAction.InternalLoadBalancerDNSName
           EnvironmentFiles:
             - !If
               - HasEnvFile

internal/pkg/deploy/cloudformation/stack/testdata/workloads/backend/https-path-alias-template.yml

@@ -80,6 +80,8 @@ Resources:
               Value: !Sub '${EnvName}'
             - Name: COPILOT_SERVICE_NAME
               Value: !Sub '${WorkloadName}'
+            - Name: COPILOT_LB_DNS
+              Value: !GetAtt EnvControllerAction.InternalLoadBalancerDNSName
           EnvironmentFiles:
             - !If
               - HasEnvFile

internal/pkg/template/templates/workloads/partials/cf/envvars-common.yml

@@ -28,13 +28,19 @@
       {{- end}}
       {{- end}}
 {{- end}}{{- end}}
-{{- if eq .WorkloadType "Load Balanced Web Service"}}
 {{- if .ALBListener}}
 - Name: COPILOT_LB_DNS
-{{- if .ImportedALB}}
+  {{- if eq .WorkloadType "Load Balanced Web Service"}}
+    {{- if .ImportedALB}}
   Value: {{.ImportedALB.DNSName}}
-{{- else}}
+    {{- else}}
   Value: !GetAtt EnvControllerAction.PublicLoadBalancerDNSName
-{{- end}}
-{{- end}}
+    {{- end}}
+  {{- else if eq .WorkloadType "Backend Service"}}
+    {{- if .ImportedALB}}
+  Value: {{.ImportedALB.DNSName}}
+    {{- else}}
+  Value: !GetAtt EnvControllerAction.InternalLoadBalancerDNSName
+    {{- end}}
+  {{- end}}
 {{- end}}