fix(Dockerfile.alpine): install wget to add support for building behind a proxy

The install-opentofu.sh script uses wget to download the binary by
default, but as the default wget binary in Alpine comes from busybox and
doesn't support HTTP proxies we're installing the actual wget binary to
work around that issue.

fixes #87

Signed-off-by: Lukas Grossar <lukas.grossar@adfinis.com>