Invalid version value when passing "alpine" to build-rootfs script

[mthalman]

Dockerfiles that pass a versionless "alpine" value to Arcade's build-rootfs script (example) fail with a message similar to the following:

 > [builder 3/3] RUN /scripts/eng/common/cross/build-rootfs.sh arm64 alpine --skipunmount:
3.092   lldb-dev (no such package):
3.092     required by: world[lldb-dev]
3.092   lttng-ust-dev (no such package):
3.092     required by: world[lttng-ust-dev]
3.092   openssl-dev (no such package):
3.092     required by: world[openssl-dev]
3.092   python3 (no such package):
3.092     required by: world[python3]
3.092   zlib-dev (no such package):
3.092     required by: world[zlib-dev]

This is due to https://github.com/dotnet/arcade/pull/15327/files#r1886908574.

[am11]

This was fixed by dotnet/arcade#15338.