docker: install ca-certificates package in base image

Resolve SSL verification issue at shippable container's git_sync stage:

shippable logs:
--------------
git_sync
- ssh-agent bash -c 'ssh-add /tmp/ssh/01_deploy; git clone https://github.com/philmd/qemu.git /root/src/github.com/philmd/qemu'
Identity added: /tmp/ssh/01_deploy (rsa w/o comment)
Cloning into '/root/src/github.com/philmd/qemu'...
fatal: unable to access 'https://github.com/philmd/qemu.git/': Problem with the SSL CA cert (path? access rights?)
retrying 1 of 3 times...

Suggested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
[AJB: fixed re-base conflict following stretch updates]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
