Add Alpine Linux support to the official install script

[MKMithun2806]

Is your feature request related to a problem? Please describe.
The official Linux install script (https://sliver.sh/install) currently only supports distributions that use apt-get, yum, or pacman. Alpine Linux (which uses apk) is not supported and exits with "Unsupported OS".

Alpine is commonly used in containers, lightweight VMs, and cloud environments due to its small footprint. Supporting it would make it easier for operators to quickly deploy Sliver on Alpine-based systems.

Describe the solution you'd like
Add detection and package installation support for Alpine Linux using apk.

Suggested changes in the install script:

• Detect Alpine via command -v apk
• Install required packages with something like:

apk add --no-cache curl git make minisign build-base

• Continue with the rest of the existing flow (download, signature verification, systemd service, operator configs, etc.)

Describe alternatives you've considered

• Manually downloading the Linux binaries from GitHub releases and setting everything up by hand
• Using the Docker image
• Building from source on Alpine

These work, but they are more friction compared to the one-liner experience available on Debian/Ubuntu/RHEL/Arch.

Additional context

• Alpine is popular for minimal and containerized deployments.
• The rest of the install script (binary download, minisign verification, systemd unit, operator config generation) should work on Alpine with minimal changes once the package manager detection is added.
• Happy to test a PR or help implement the apk branch if needed.