chore: change default thumbnail concurrency and auto-detect container core count (#9981)

* feat: automatically detect amount of CPU cores and allow overriding with CPU_CORES env var

* chore: change default thumbnail concurrency to 3

docker/scripts/get-cpus.sh

@@ -0,0 +1,49 @@
+#!/bin/sh
+
+set -eu
+
+LOG_LEVEL="${IMMICH_LOG_LEVEL:='info'}"
+
+logDebug() {
+    if [ "$LOG_LEVEL" = "debug" ] || [ "$LOG_LEVEL" = "verbose" ]; then
+        echo "DEBUG: $1" >&2
+    fi
+}
+
+if [ -f /sys/fs/cgroup/cgroup.controllers ]; then
+    logDebug "cgroup v2 detected."
+    if [ -f /sys/fs/cgroup/cpu.max ]; then
+        read -r quota period </sys/fs/cgroup/cpu.max
+        if [ "$quota" = "max" ]; then
+            logDebug "No CPU limits set."
+            unset quota period
+        fi
+    else
+        logDebug "/sys/fs/cgroup/cpu.max not found."
+    fi
+else
+    logDebug "cgroup v1 detected."
+
+    if [ -f /sys/fs/cgroup/cpu/cpu.cfs_quota_us ] && [ -f /sys/fs/cgroup/cpu/cpu.cfs_period_us ]; then
+        quota=$(cat /sys/fs/cgroup/cpu/cpu.cfs_quota_us)
+        period=$(cat /sys/fs/cgroup/cpu/cpu.cfs_period_us)
+
+        if [ "$quota" = "-1" ]; then
+            logDebug "No CPU limits set."
+            unset quota period
+        fi
+    else
+        logDebug "/sys/fs/cgroup/cpu/cpu.cfs_quota_us or /sys/fs/cgroup/cpu/cpu.cfs_period_us not found."
+    fi
+fi
+
+if [ -n "${quota:-}" ] && [ -n "${period:-}" ]; then
+    cpus=$((quota / period))
+    if [ "$cpus" -eq 0 ]; then
+        cpus=1
+    fi
+else
+    cpus=$(grep -c processor /proc/cpuinfo)
+fi
+
+echo "$cpus"