Makefile: add spk and pushspk targets for iterative Synology development

Change-Id: I97319d14917aa2b00ff72a7b73b3db79ea8392b7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f