Short answer: No, it is not pre-assumed that you’ll have knowledge on Linux. Not everyone uses Linux. But this answer is Yes, if you are already 3-5 years into working. At this stage, it is good to have an understanding about Linux commands. But you don’t need to be a pro in these topics to be only doing front-end.
Is this the first time you are learning programming and you chose to start with HTML, CSS, JS & React? Then focus on these things itself. If your goal is to be a frontend developer then HTML, CSS, JS are the foundation and basics, so get a decent understanding of these first.
When you have worked on a few applications and you want to expand your knowledge base then you can think about what can be done beyond frontend? Or how does frontend work together with backend.
If you are interested in understanding frontend + backed, this is where Docker, Kubernetes (K8s) come as an extension to your existing knowledge base.
If you are entry level, then the idea is to be strong with core programming concepts. It does not matter which tools or frameworks, but since you practice programming you would anyways familiarise with one langauge/framework and this is enough. What is important is how you approach a problem, how you learn and how quickly you can learn new things.
If you are 2-3 years into programming and working, then basic understanding of Docker/K8s is nice (not a must) since this depends on your team/project/work environment.
To deploy a frontend application, we usually setup a web server. And when the applications are built in a CI/CD setup this is where Docker comes handy and you run them via Docker/K8s in one of the cloud providers.
If you get to work on a web application focused development then you start with frontend tools and frameworks and based on your interest you can deep dive further into automating things, improving frontend tooling etc. This is where Docker/K8s concepts come handy and also based on your interest if you want to proceed this route or not. If you don’t like this, then you can also look into writing better tests, automating them, improving application’s performance, improving CSS page load, etc. etc.
About learning Linux, it is more about being able to do simple commands like copy, move, delete, cd, etc in bash environment. Again, if you are interested you can deep dive into this.
The learning itself is a process, it is ok to be overwhelmed. This is where you ask questions with communities or mentors. I have written a post on “how to learn”.