fix: Ensure PCNTL extension is always installed in Docker

[Wirone]

We need signals support both in local development and in official releases.

Related to #7783, but does not completely resolve it.

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Explanation:

• we need pcntl extension both for official releases and local development, so it's extracted to the lowest base layer
• Composer is required for local development and for preparing Docker releases, so it's extracted to base-dev layer
• vendor layer is only an intermediate layer used to COPY --from to copy pre-installed vendor packages to the Docker release
• dist is a layer with official Docker releases, built on top of base (with pcntl), with vendors copied from vendor
• dev is a top layer for local development, built od top of base-dev (with pcntl and Composer)

[Wirone]

FYI: we currently have problem with ctrl+C signal anyway (does not stop the command even when run natively on the host), it's not related to Docker and must be fixed independently. Will create issue for that and will look at it.

Edit: it seems like killing the processes makes my terminal working bad and signals did not work for native execution, but when I've checked in fresh terminal (PHAR distributions, native call from the source code, call inside Docker container) and ctrl+C is handled properly. So it seems there's only problem when running via docker run.

[coveralls]

coverage: 94.753%. remained the same
when pulling b8826ae on Wirone:codito/docker-with-signals-support
into aafe887 on PHP-CS-Fixer:master.

[julienfalque]

LGTM, although I didn't actually test the changes.