Add support to register additional CA cert to cacerts

[robertoschwald]

If using a ca cert which is not known to the JDK, you get error gadget.common.error.500
So if using e.g. a LetsEncrypt SSL cert on your frontend proxy server, the "DST Root CA X3" is not known which causes mentioned error message.

It would be super cool if the Image itself would be able to register an additional CA cert by providing e.g.

ADDITIONAL_CA_CERT="myaddtionalCaCert.pem"

The entrypoint then should be able to automatically register the cert before firing up Jira like so:
#14 (comment)