compute cpu and memory stats for docker on windows (#653)

The Docker daemon's API returns different values on Windows and non-Windows systems. This impacts
the calculation of `cpuPct` and `usedMemory` for each container. The systems are disciminate on the
`Server` header send by the server. Uses the unix stats calculation in case the header is not set.

`docker stats` implementation for reference:
https://github.com/docker/cli/blob/master/cli/command/container/stats_helpers.go#L100

Co-authored-by: Benoit VIRY <benoit.viry@cgx-group.com>

beszel/internal/entities/container/container.go

@@ -28,7 +28,7 @@ type ApiInfo struct {
 // Docker container resources from /containers/{id}/stats
 type ApiStats struct {
 	// Common stats
-	// Read    time.Time `json:"read"`
+	Read time.Time `json:"read"`
 	// PreRead time.Time `json:"preread"`
 
 	// Linux specific stats, not populated on Windows.
@@ -36,7 +36,7 @@ type ApiStats struct {
 	// BlkioStats BlkioStats `json:"blkio_stats,omitempty"`
 
 	// Windows specific stats, not populated on Linux.
-	// NumProcs uint32 `json:"num_procs"`
+	NumProcs uint32 `json:"num_procs"`
 	// StorageStats StorageStats `json:"storage_stats,omitempty"`
 	// Networks request version >=1.21
 	Networks map[string]NetworkStats
@@ -101,7 +101,7 @@ type MemoryStats struct {
 	// // peak committed bytes
 	// CommitPeak uint64 `json:"commitpeakbytes,omitempty"`
 	// // private working set
-	// PrivateWorkingSet uint64 `json:"privateworkingset,omitempty"`
+	PrivateWorkingSet uint64 `json:"privateworkingset,omitempty"`
 }
 
 type MemoryStatsStats struct {
@@ -131,4 +131,5 @@ type Stats struct {
 	NetworkRecv float64      `json:"nr"`
 	PrevCpu     [2]uint64    `json:"-"`
 	PrevNet     prevNetStats `json:"-"`
+	PrevRead    time.Time    `json:"-"`
 }