refactor: replace su-exec/gosu with setpriv

[xoxys]

The used tool gosu/su-exec creates a lot of noise in the security scanning (even if the CVE's are not applicable). As the same functionality can be achieved by OS default tools this PR replaces gosu with setpriv.

usr/bin/su-exec (gobinary)
==========================
Total: 4 (UNKNOWN: 1, LOW: 0, MEDIUM: 3, HIGH: 0, CRITICAL: 0)

+--------------------------------+---------------------+----------+------------------------------------+-----------------------------------+----------------------------------------------+
|            LIBRARY             |  VULNERABILITY ID   | SEVERITY |         INSTALLED VERSION          |           FIXED VERSION           |                    TITLE                     |
+--------------------------------+---------------------+----------+------------------------------------+-----------------------------------+----------------------------------------------+
| github.com/opencontainers/runc | CVE-2021-43784      | MEDIUM   | v1.0.1                             | 1.1.0                             | runc: integer overflow in                    |
|                                |                     |          |                                    |                                   | netlink bytemsg length field                 |
|                                |                     |          |                                    |                                   | allows attacker to override...               |
|                                |                     |          |                                    |                                   | -->avd.aquasec.com/nvd/cve-2021-43784        |
+                                +---------------------+          +                                    +-----------------------------------+----------------------------------------------+
|                                | CVE-2022-24769      |          |                                    | v1.1.2                            | moby: Default inheritable                    |
|                                |                     |          |                                    |                                   | capabilities for linux                       |
|                                |                     |          |                                    |                                   | container should be empty                    |
|                                |                     |          |                                    |                                   | -->avd.aquasec.com/nvd/cve-2022-24769        |
+                                +---------------------+----------+                                    +-----------------------------------+----------------------------------------------+
|                                | GHSA-v95c-p5hm-xq8f | UNKNOWN  |                                    | 1.1.0                             | An attacker with partial control             |
|                                |                     |          |                                    |                                   | over the bind mount sources of a...          |
|                                |                     |          |                                    |                                   | -->github.com/advisories/GHSA-v95c-p5hm-xq8f |
+--------------------------------+---------------------+----------+------------------------------------+-----------------------------------+----------------------------------------------+
| golang.org/x/sys               | CVE-2022-29526      | MEDIUM   | v0.0.0-20210817142637-7d9622a276b7 | 0.0.0-20220412211240-33da011f77ad | golang: syscall: faccessat                   |
|                                |                     |          |                                    |                                   | checks wrong group                           |
|                                |                     |          |                                    |                                   | -->avd.aquasec.com/nvd/cve-2022-29526        |
+--------------------------------+---------------------+----------+------------------------------------+-----------------------------------+----------------------------------------------+

Reference: https://github.com/tianon/gosu#setpriv

[xoxys]

@d7oc FYI