Dockerfile: remove unnecessary fly build flags
Dockerfile: remove unnecessary fly build flags

netgo/osusergo aren't needed now that we build with CGO_ENABLED=0, and
-a shouldn't be needed either way (seems like it'd just slow down
compilation)

Signed-off-by: Alex Suraci <925ba519bbf1a06224928b692ef88b93aee946af@gmail.com>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse