Most start up companies have open source at the core of their philosophy, so it naturally follows that people working at such companies will have a rich open source portfolio. For instance, today I contributed a fix to a plugin we're using. Making open source contributions is just part of my normal workflow.

If you are working at a company that doesn't value open source now, and you don't contribute to open source, it will be hard to transition to a job that does value open source contributions.

You mention your current job is in banking. If you were to apply to another banking job, you'd probably have a better chance of getting the position than someone who has a rich open source portfolio, but no experience in the industry.

Having a GitHub account with x projects is just the new has x years of experience in Java.