Add multi-package Alpine packaging

packaging/alpine/local/greptimedb/README.md

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+# GreptimeDB Alpine Package
+
+This package builds `greptime` from the upstream GreptimeDB source release.
+
+The old `Dockerfile` and `Makefile` in this directory are reference material for
+the previous manual build. The APKBUILD uses the same build shape directly:
+
+```sh
+cargo build --release --locked --bin greptime --features servers/dashboard
+```
+
+## Package Commands
+
+Refresh the source checksum after changing `pkgver`:
+
+```sh
+ALPINE_PACKAGE=greptimedb mise run apk:checksum
+```
+
+Build only this package:
+
+```sh
+ALPINE_PACKAGE=greptimedb mise run apk:build
+```
+
+Install-test an existing build without recompiling:
+
+```sh
+ALPINE_PACKAGE=greptimedb SKIP_BUILD=1 mise run apk:test-install
+```
+
+The full build can take close to an hour. Do not run it as part of lightweight
+metadata or script checks.