release/dist/qnap: add qnap target builder

Creates new QNAP builder target, which builds go binaries then uses
docker to build into QNAP packages. Much of the docker/script code
here is pulled over from https://github.com/tailscale/tailscale-qpkg,
with adaptation into our builder structures.

The qnap/Tailscale folder contains static resources needed to build
Tailscale qpkg packages, and is an exact copy of the existing folder
in the tailscale-qpkg repo.

Builds can be run with:
```
sudo ./tool/go run ./cmd/dist build qnap
```

Updates tailscale/tailscale-qpkg#135

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

release/dist/qnap/targets.go

@@ -0,0 +1,75 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package qnap
+
+import "tailscale.com/release/dist"
+
+// Targets defines the dist.Targets for QNAP devices.
+//
+// If privateKeyPath and certificatePath are both provided non-empty,
+// these targets will be signed for QNAP app store release with built.
+func Targets(privateKeyPath, certificatePath string) []dist.Target {
+	var signerInfo *signer
+	if privateKeyPath != "" && certificatePath != "" {
+		signerInfo = &signer{privateKeyPath, certificatePath}
+	}
+	return []dist.Target{
+		&target{
+			arch: "x86",
+			goenv: map[string]string{
+				"GOOS":   "linux",
+				"GOARCH": "386",
+			},
+			signer: signerInfo,
+		},
+		&target{
+			arch: "x86_ce53xx",
+			goenv: map[string]string{
+				"GOOS":   "linux",
+				"GOARCH": "386",
+			},
+			signer: signerInfo,
+		},
+		&target{
+			arch: "x86_64",
+			goenv: map[string]string{
+				"GOOS":   "linux",
+				"GOARCH": "amd64",
+			},
+			signer: signerInfo,
+		},
+		&target{
+			arch: "arm-x31",
+			goenv: map[string]string{
+				"GOOS":   "linux",
+				"GOARCH": "arm",
+			},
+			signer: signerInfo,
+		},
+		&target{
+			arch: "arm-x41",
+			goenv: map[string]string{
+				"GOOS":   "linux",
+				"GOARCH": "arm",
+			},
+			signer: signerInfo,
+		},
+		&target{
+			arch: "arm-x19",
+			goenv: map[string]string{
+				"GOOS":   "linux",
+				"GOARCH": "arm",
+			},
+			signer: signerInfo,
+		},
+		&target{
+			arch: "arm_64",
+			goenv: map[string]string{
+				"GOOS":   "linux",
+				"GOARCH": "arm64",
+			},
+			signer: signerInfo,
+		},
+	}
+}