Dockerfile: add missing curl dependency

Without it we were failing to download fstrm tarball,
and the shown errors were very confusing.
Tested building an image locally.
5 jobs for <span class="ref-name">docker</span> in 32 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
canceled #73085
docker linux amd64
build:linux:amd64

passed #19165
docker linux amd64
build:linux:amd64

00:00:36

 
  Test
passed #19167
docker linux amd64
deckard:linux:amd64

00:00:32

canceled #19168
docker linux amd64 allowed to fail
respdiff:linux:amd64

00:00:15

passed #19166
docker linux amd64
test:linux:amd64

00:00:22