We only need updated podman on `build`. `test` and `release` can use image
provided container engine binaries.

The base images will be now built in
[another repository](https://github.com/searxng/base).

Let container instance administrators to add custom certificates:

  https://docs.searxng.org/admin/installation-docker.html#custom-certificates

Closes https://github.com/searxng/searxng/issues/5206

Docker buildx outputs the following error:

  variable expansion is not supported for --from, define a new stage with FROM
  using ARG from global scope as a workaround.

Also force BuildKit extension to be installed, legacy build is no longer
supported.

Closes https://github.com/searxng/searxng/issues/5219

We can leverage the immutable nature of containers to add additional
optimizations.

No debugging or tinkering inside containers, so stripping all unused symbols
inside `venv` should be fine. We are also going to compile the bytecode
ourselves to modify some parameters related to reproducibility.

With these small changes, we have reduced the `venv` layer size by 10MB~

* [enh] container: reproducible layers

We are not aiming for reproducibility compliance, but we look to make most
builder layers reproducible without caching at least for a short period of time
(until the builder's base image changes or the child dependencies of a
requirements.txt package are updated).

This feature is only available on Podman.

This targets https://github.com/searxng/searxng/pull/5086 main goal.

* [fix] misc: apply suggestions

Suggested: https://github.com/searxng/searxng/pull/5222#discussion_r2364630496
Suggested: https://github.com/searxng/searxng/pull/5222#discussion_r2364630511

* [enh] container: prevent useless layer

We get some good stuff without uvloop, 13MB~ less of dependencies, 3
minutes of build time for armv7 saved, and we are one step closer to NT
compatibility. Although it's true that theoretically the raw performance
have worsened on network side (we only used uvloop for that), the latest
cpython versions have been improving on asyncio performance.

This commit replaces `pip` in container builds with `uv` pip compat
with a 1:1 parity. The only thing that changes is the installation speed of the
wheels, which seems to be considerably faster, although I haven't been able to
properly quantify this yet.

uv also gives us more tools to manage the cache. We can revert the prior cache
changes in `container.yml` as we won't have duplicated wheels anymore.

https://github.com/searxng/searxng/pull/5192