fix: use usr/bin/env to locate interpreter

[rbuckland]

Inside python:3.10-slim, the path to python3 is at /usr/local/bin/python3

Which causes the running of the systemctl replacement in this (and maybe other docker containers fail)

trying to launch via bash (command prompt)

root@8fa696f7b7a2:/build# /usr/bin/systemctl 
bash: /usr/bin/systemctl: cannot execute: required file not found

This is it working, specifying the python3 interpreter directly.

root@8fa696f7b7a2:/build# which python3
/usr/local/bin/python3

root@8fa696f7b7a2:/build# python3 /usr/bin/systemctl 
apt-daily-upgrade.service       loaded inactive dead    Daily apt upgrade and clean activities
apt-daily-upgrade.timer loaded unknown dead     Daily apt upgrade and clean activities
apt-daily.service       loaded inactive dead    Daily apt download activities
apt-daily.timer loaded unknown dead     Daily apt download activities
dpkg-db-backup.service  loaded inactive dead    Daily dpkg database backup service
dpkg-db-backup.timer    loaded unknown dead     Daily dpkg database backup timer
e2scrub@.service        loaded inactive dead    Online ext4 Metadata Check for 
e2scrub_all.service     loaded inactive dead    Online ext4 Metadata Check for All Filesystems
e2scrub_all.timer       loaded unknown dead     Periodic ext4 Online Metadata Check for All Filesystems
e2scrub_fail@.service   loaded inactive dead    Online ext4 Metadata Check Failure Reporting for 
e2scrub_reap.service    loaded inactive dead    Remove Stale Online ext4 Metadata Check Snapshots
fstrim.service  loaded inactive dead    Discard unused blocks on filesystems from /etc/fstab
fstrim.timer    loaded unknown dead     Discard unused blocks once a week
hwclock.sh.service      loaded inactive dead
pam_namespace.service   loaded inactive dead    Make sure parent directories configured in /etc/security/namespace.conf for polyinstantiation exist

This is due to this issue

[rbuckland]

This was using FROM python:3.10-slim