Setting Up a bridge

apk add bridge-utils

vi /etc/network/interfaces

auto br0
iface br0 inet static
pre-up brctl addbr br0
pre-up echo 0 > /proc/sys/net/bridge/bridge-nf-call-arptables
pre-up echo 0 > /proc/sys/net/bridge/bridge-nf-call-iptables
pre-up echo 0 > /proc/sys/net/bridge/bridge-nf-call-ip6tables
address 192.168.0.253
netmask 255.255.255.0
gateway 192.168.0.254
post-down brctl delbr br0

auto eth0
iface eth0 inet manual
up ip link set $IFACE up
up brctl addif br0 $IFACE
down brctl delif br0 $IFACE || true
down ip link set $IFACE down

auto eth1
iface eth1 inet manual
up ip link set $IFACE up
up brctl addif br0 $IFACE
down brctl delif br0 $IFACE || true
down ip link set $IFACE down
...

That way you create br0 with: ifup br0, and you can add/remove individual interfaces to the bridge with ifup eth0, ifdown eth0.