Telnet disappears from container if you rebuild

[AartBluestoke]

because alpine-java:8 has dropped several packages
alpine-java:8 has been updated, as of 10 days ago, without releasing a new version.
The underlying alpine has moved forward in versions and several included packages are no longer present in the underlying busybox (eg: telnet) having being moved to -extras.
as of gliderlabs/docker-alpine#397
(i was rebuilding this from source so that i could use a newer liquibase version)

[AartBluestoke]

"apk add busybox-extras" is all that is needed, although i'm not sure if it is needed here, or just documented so that it can be added on demand in downstream packages.