chore: fix copying of variables into Docker (#1203)
chore: fix copying of variables into Docker (#1203)

Fixes `set -u` errors, and prevents variables being set to undefined values

Change-type: patch
resin-io/etcher,resin-io/etcher,resin-io/herostratus,resin-io/etcher,resin-io/herostratus,resin-io/resin-etcher,resin-io/etcher,resin-io/resin-etcher,resin-io/etcher