helm: `helm repo add` does not work with corporate certs

Output of helm version:

$ helm version
Client: &version.Version{SemVer:"v2.8.2", GitCommit:"a80231648a1473929271764b920a8e346f6de844", GitTreeState:"clean"}
Server: &version.Version{SemVer:"v2.8.2", GitCommit:"a80231648a1473929271764b920a8e346f6de844", GitTreeState:"clean"}

Output of kubectl version:

$ kubectl version
Client Version: version.Info{Major:"1", Minor:"10", GitVersion:"v1.10.0", GitCommit:"fc32d2f3698e36b93322a3465f63a14e9f0eaead", GitTreeState:"clean", BuildDate:"2018-03-27T00:13:02Z", GoVersion:"go1.9.4", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"", Minor:"", GitVersion:"v1.9.4", GitCommit:"bee2d1505c4fe820744d26d41ecd3fdd4a3d6546", GitTreeState:"clean", BuildDate:"2018-03-21T21:48:36Z", GoVersion:"go1.9.1", Compiler:"gc", Platform:"linux/amd64"}

Cloud Provider/Platform (AKS, GKE, Minikube etc.):

minikube

Description:

I can curl a resource file:

$ curl --cacert ~/.cert/corporate-cert.pem https://lightbend.github.io/helm-charts/index.yaml
apiVersion: v1
entries:
  enterprise-suite:
  - apiVersion: v1
    created: 2018-03-07T18:46:14.935560601Z
    description: Lightbend Enterprise Suite
    # ...

but I cannot add with --ca-file:

$ helm repo add --ca-file ~/.cert/corporate-cert.pem lightbend-helm-charts https://lightbend.github.io/helm-charts

Error: Looks like "https://lightbend.github.io/helm-charts" is not a valid chart repository or cannot be reached: Get https://lightbend.github.io/helm-charts/index.yaml: x509: certificate signed by unknown authority

I’ve added this cert to my minikube VM /etc/ssl/certs/ca-certificates.crt, but I still cannot add the repository.