Update README.md: su-exec no longer needed on Alpine 3.8+

[am97]

Hello,

The test on the README.md is no longer true on Alpine 3.8+

$ docker run --rm -it alpine:3.8 su postgres -c 'ps aux'
PID   USER     TIME  COMMAND
    1 postgres  0:00 ps aux

Seems to come from this Busybox commit: https://git.busybox.net/busybox/commit/?h=1_28_stable&id=1e3e2ccd5dd280371c9ca29c0e0304a0d40592af

On Debian 12, there are still forks that can be avoided with su-exec. Here is a simple test:

Dockerfile:

FROM debian:bookworm

RUN useradd postgres && \
    apt-get update && \
    apt-get install --no-install-recommends -y procps && \
    rm -rf /var/lib/apt/lists/*

Test:

$ docker run --rm -it bookworm_demo su postgres -c 'ps aux'
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1  7.6  0.0   4120  2560 pts/0    Ss+  16:14   0:00 su postgres -c ps aux
postgres       7  0.0  0.0   2576  1536 ?        Ss   16:14   0:00 sh -c ps aux
postgres       8  0.0  0.0   8084  3840 ?        R    16:14   0:00 ps aux

The Debian case may be more relevant for su-exec users, I can modify this PR to put the previous example instead of Alpine if you wish.