Description

This escape-hatch was added in docker 28.0, and can be removed in 29.0 (or possibly a later "minor" release of 28.x)

Related to:

• Remove iptables rule for SCTP checksum fixup #47952
• By-default, don't create an iptables rule to checksum SCTP #48149

If the escape hatch DOCKER_IPTABLES_SCTP_CHECKSUM=1 isn't needed - remove it, and the code to set up the checksum mangling rule.