ExecProbes should be able to do simple env var substitution

For containers that don't have bash, we should support env substitution
like we do on command and args. However, without major refactoring
valueFrom is not supportable from inside the prober. For now, implement
substitution based on hardcoded env and leave TODOs for future work.

pkg/kubelet/container/helpers.go

@@ -119,10 +119,33 @@ func EnvVarsToMap(envs []EnvVar) map[string]string {
 	for _, env := range envs {
 		result[env.Name] = env.Value
 	}
+	return result
+}
+
+// V1EnvVarsToMap constructs a map of environment name to value from a slice
+// of env vars.
+func V1EnvVarsToMap(envs []v1.EnvVar) map[string]string {
+	result := map[string]string{}
+	for _, env := range envs {
+		result[env.Name] = env.Value
+	}
 
 	return result
 }
 
+// ExpandContainerCommandOnlyStatic substitutes only static environment variable values from the
+// container environment definitions. This does *not* include valueFrom substitutions.
+// TODO: callers should use ExpandContainerCommandAndArgs with a fully resolved list of environment.
+func ExpandContainerCommandOnlyStatic(containerCommand []string, envs []v1.EnvVar) (command []string) {
+	mapping := expansion.MappingFuncFor(V1EnvVarsToMap(envs))
+	if len(containerCommand) != 0 {
+		for _, cmd := range containerCommand {
+			command = append(command, expansion.Expand(cmd, mapping))
+		}
+	}
+	return command
+}
+
 func ExpandContainerCommandAndArgs(container *v1.Container, envs []EnvVar) (command []string, args []string) {
 	mapping := expansion.MappingFuncFor(EnvVarsToMap(envs))