Running pod with securityContext does not allow private CA installation

The permissions necessary to install a private CA via certsSecretName are not available when running with pod securityContext set.

Commit: https://gitlab.com/charts/gitlab-runner/commit/af38d2aa2a69378440ab722622eb1530732976a7#c4d1b3e8c0326ad0a803a3f2e1467107af5d0027_21_21

Logs:

gitlab-runner-gitlab-runner	Jun 4, 2018, 4:08:32 PM	[0;33mWARNING: Running in user-mode. [0;m
gitlab-runner-gitlab-runner	Jun 4, 2018, 4:08:32 PM	Failed to open temporary file /etc/ssl/certs/bundleXXXXXX for ca bundle
gitlab-runner-gitlab-runner	Jun 4, 2018, 4:08:32 PM	cp: can't create '/usr/local/share/ca-certificates/ca.crt': Permission denied
gitlab-runner-gitlab-runner	Jun 4, 2018, 4:08:32 PM	Updating CA certificates...