Add ca-certificates to Dockerfile.hsm

[andrewmzhang]

This addition is necessary if you want to run the smallstep/step-ca:hsm container in linked mode. Without ca-certificates, the container is unable to verify https certificates of and connect to smallstep.com

Name of feature:

Adds ca-certificates package to smallstep/step-ca:hsm docker image.

Pain or issue this feature alleviates:

As of time of pull request, the smallstep/step-ca:hsm docker image cannot run step-ca in linked mode, because it cannot verify the https certificate of and connect to smallstep.com. This change will add the typical public root CAs to the container's truststore.

Why is this important to the project (if not answered above):

I want to run the container in linked mode

Is there documentation on how to use this feature? If so, where?

N/A

In what environments or workflows is this feature supported?

N/A

In what environments or workflows is this feature explicitly NOT supported (if any)?

N/A

Supporting links/other PRs/issues:

💔Thank you!