[BUG] Missing existing containers mount ns in cache

[AlonZivony]

Prerequisites

• [x] This affects latest released version.
• [x] This affects current development tree (origin/HEAD).
• [x] There isn't an issue describing the bug.

Select one OR another:

• [x] I'm going to create a PR to solve this (assign to yourself).
• [ ] Someone else should solve this.

Bug description

Tracee has the member pidsInMntns which cache PIDs in each mount namespace, where the first one is the entry-point process. Using this cache we access files inside the different mount namespaces using procfs. However, the cache is filled using the sched_process_exec event alone. As a result, it's missing mount namespaces which existed before tracee initialization (as long as no process was executed there). This result to many errors and reduction in tracee reliability.

[AlonZivony]

See #2036 for the source discussion resulted this issue.

[AlonZivony]

Can be extracted from the ExistingContainer event