Add systemd files

systemd/cert-renewer@.service

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+[Unit]
+Description=Certificate renewer for %I
+After=network-online.target
+Documentation=https://smallstep.com/docs/step-ca/certificate-authority-server-production
+StartLimitIntervalSec=0
+
+[Service]
+Type=oneshot
+User=root
+
+Environment=STEPPATH=/etc/step-ca \
+            CERT_LOCATION=/etc/step/certs/%i.crt \
+            KEY_LOCATION=/etc/step/certs/%i.key
+
+; ExecStartPre checks if the certificate is ready for renewal,
+; based on the exit status of the command.
+; (In systemd 243 and above, you can use ExecCondition= here.)
+ExecStartPre=/usr/bin/bash -c \
+  'step certificate inspect $CERT_LOCATION --format json --roots "$STEPPATH/certs/root_ca.crt" | \
+  jq -e "(((.validity.start | fromdate) + \
+          ((.validity.end | fromdate) - (.validity.start | fromdate)) * 0.66) \
+           - now) <= 0" > /dev/null'
+
+; ExecStart renews the certificate, if ExecStartPre was successful.
+ExecStart=/usr/bin/step ca renew --force $CERT_LOCATION $KEY_LOCATION
+
+; Try to reload or restart the systemd service that relies on this cert-renewer
+ExecStartPost=/usr/bin/bash -c 'systemctl --quiet is-enabled %i && systemctl try-reload-or-restart %i'
+
+[Install]
+WantedBy=multi-user.target

systemd/cert-renewer@.timer

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+[Unit]
+Description=Certificate renewal timer for %I
+Documentation=https://smallstep.com/docs/step-ca/certificate-authority-server-production
+
+[Timer]
+Persistent=true
+
+; Run the timer unit every 5 minutes.
+OnCalendar=*:1/5
+
+; Always run the timer on time.
+AccuracySec=1us
+
+; Add jitter to prevent a "thundering hurd" of simultaneous certificate renewals.
+RandomizedDelaySec=5m
+
+[Install]
+WantedBy=timers.target

systemd/step-ca.service

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+[Unit]
+Description=step-ca service
+Documentation=https://smallstep.com/docs/step-ca
+Documentation=https://smallstep.com/docs/step-ca/certificate-authority-server-production
+After=network-online.target
+Wants=network-online.target
+StartLimitIntervalSec=30
+StartLimitBurst=3
+ConditionFileNotEmpty=/etc/step-ca/config/ca.json
+ConditionFileNotEmpty=/etc/step-ca/password.txt
+
+[Service]
+Type=simple
+User=step
+Group=step
+Environment=STEPPATH=/etc/step-ca
+WorkingDirectory=/etc/step-ca
+ExecStart=/usr/local/bin/step-ca config/ca.json --password-file password.txt
+ExecReload=/bin/kill --signal HUP $MAINPID
+Restart=on-failure
+RestartSec=5
+TimeoutStopSec=30
+StartLimitInterval=30
+StartLimitBurst=3
+
+; Process capabilities & privileges
+AmbientCapabilities=CAP_NET_BIND_SERVICE
+CapabilityBoundingSet=CAP_NET_BIND_SERVICE
+SecureBits=keep-caps
+NoNewPrivileges=yes
+
+; Sandboxing
+ProtectSystem=full
+RestrictNamespaces=true
+RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6
+PrivateTmp=true
+ProtectClock=true
+ProtectControlGroups=true
+ProtectKernelTunables=true
+ProtectKernelLogs=true
+ProtectKernelModules=true
+LockPersonality=true
+RestrictSUIDSGID=true
+RemoveIPC=true
+RestrictRealtime=true
+; confirmed this works, even with YubiKey PIV, and presumably with YubiHSM2:
+PrivateDevices=true
+MemoryDenyWriteExecute=true
+ReadWriteDirectories=/etc/step-ca/db
+
+[Install]
+WantedBy=multi-user.target