client: interpolate driver configurations

Also add missing SetDriverNetwork calls.

client/allocrunner/taskrunner/task_runner.go

@@ -462,8 +462,8 @@ func (tr *TaskRunner) runDriver() error {
 	// TODO(nickethier): make sure this uses alloc.AllocatedResources once #4750 is rebased
 	taskConfig := tr.buildTaskConfig()
 
-	// TODO: load variables
 	evalCtx := &hcl.EvalContext{
+		Variables: tr.envBuilder.Build().AllValues(),
 		Functions: shared.GetStdlibFuncs(),
 	}
 

client/allocrunner/taskrunner/task_runner_getters.go

@@ -61,11 +61,15 @@ func (tr *TaskRunner) getDriverHandle() *DriverHandle {
 	return tr.handle
 }
 
-// setDriverHanlde sets the driver handle and creates a new result proxy.
+// setDriverHanlde sets the driver handle, creates a new result proxy, and
+// updates the driver network in the task's environment.
 func (tr *TaskRunner) setDriverHandle(handle *DriverHandle) {
 	tr.handleLock.Lock()
 	defer tr.handleLock.Unlock()
 	tr.handle = handle
+
+	// Update the environment's driver network
+	tr.envBuilder.SetDriverNetwork(handle.net)
 }
 
 func (tr *TaskRunner) clearDriverHandle() {

client/driver/env/env.go

@@ -12,6 +12,7 @@ import (
 	"github.com/hashicorp/nomad/helper"
 	hargs "github.com/hashicorp/nomad/helper/args"
 	"github.com/hashicorp/nomad/nomad/structs"
+	"github.com/zclconf/go-cty/cty"
 )
 
 // A set of environment variables that are exported by each driver.
@@ -159,6 +160,20 @@ func (t *TaskEnv) All() map[string]string {
 	return m
 }
 
+// AllValues is a map of the task's environment variables and the node's
+// attributes with cty.Value (String) values.
+func (t *TaskEnv) AllValues() map[string]cty.Value {
+	m := make(map[string]cty.Value, len(t.EnvMap)+len(t.NodeAttrs))
+	for k, v := range t.EnvMap {
+		m[k] = cty.StringVal(v)
+	}
+	for k, v := range t.NodeAttrs {
+		m[k] = cty.StringVal(v)
+	}
+
+	return m
+}
+
 // ParseAndReplace takes the user supplied args replaces any instance of an
 // environment variable or Nomad variable in the args with the actual value.
 func (t *TaskEnv) ParseAndReplace(args []string) []string {