Sql Server – Multiple errors regarding to memory in error log

MSSQL-Server 2017 Express Edition runs on my Linux server.
The server has 4GB RAM.
The total size of DB is 300MB.

My website and the sqlserver are on the same server, it’s a small website with max of 400-300 visitors a day. Memory usage in linux looks fine, mssql takes only 20% memory:

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Some additional sql memory information:
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Maximum server memory is set to 2147483647 which is the default I guess.

The errors I get:

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After restarting the mssql-service there are no problems for something like 2 days and then it repeats it self.

I would happily get an answer that will solve this for me, but I would also like to get a resource where I can learn about memory allocation/cache/pages/ related to sql server because I am not a sql expert, I am a programmer.