build(docker): adding libssl-dev, upstream image reference pinned to debian:buster

[michalschott]

What type of PR is this?
/kind bug

Any specific area of the project related to this PR?
/area build

What this PR does / why we need it:
This PR updates base image with libssl-dev package required to compile kernel module.
It also updates upstream container reference, since recently Buster becomes new stable and container refuses to build.

Which issue(s) this PR fixes:

Fixes #1718

Special notes for your reviewer:

Since Debian Bullseye become new stable, it is also required to update base image reference.

Does this PR introduce a user-facing change?:

NONE

[poiana]

Welcome @michalschott! It looks like this is your first PR to falcosecurity/falco 🎉

[leogr]

/milestone 0.30.0

[leogr]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: leogr, michalschott

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• docker/OWNERS [leogr]

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[poiana]

LGTM label has been added.

Git tree hash: e77469322f06b9fd45c45e094e10a44a0cf19d64

[leogr]

Firstly, thank you for this PR! 👍
Secondly, I forgot to add some thoughts regarding this PR, so here I'm 😺

Pinning debian:buster is ok for me as a temporary solution until we rethink the Dockerfile a bit and make it work with the latest Debian or some other way (as we discussed during yesterday's community call). Moreover, this fix will help with the CI failures we are encountering on the master branch. That is even more important if we consider that the next release is approaching (otherwise we won't be able to release if the container image refused to build).