Docker: fix Dockerfile for demo container

libstdc++ was missing in the Alpine image.
At the same time, I've enabled DNSSEC validation, DNS-over-TLS, HTTP interface
and added explanatory message.
14 jobs for dockerfile in 8 minutes and 18 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build
passed #102157
docker linux amd64
build:clang:linux:amd64

00:00:56

passed #102156
docker linux amd64
build:linux:amd64

00:00:40

 
  Test
passed #102163
docker linux amd64
deckard:linux:amd64

00:07:06

passed #102162
docker linux amd64
installcheck:linux:amd64

00:00:33

passed #102164
docker linux amd64
installcheck:valgrind:linux:amd64

00:01:01

passed #102159
docker
lint:c

00:02:46

failed #102160
docker allowed to fail
lint:clang-scan-build

00:00:38

passed #102158
docker
lint:lua

00:00:18

passed #102167
docker linux amd64
respdiff:iter:tcp:linux:amd64

00:03:12

passed #102168
docker linux amd64
respdiff:iter:tls:linux:amd64

00:02:37

passed #102166
docker linux amd64
respdiff:iter:udp:linux:amd64

00:02:25

passed #102161
docker linux amd64
test:linux:amd64

00:01:23

passed #102165
docker linux amd64
test:linux:amd64:valgrind

00:01:25

 
  Coverage
passed #102169
docker linux amd64
coverage

00:00:14

68.5%
 
Name Stage Failure
failed
lint:clang-scan-build Test
                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4 warnings generated.
CCLD daemon/kresd
scan-build: 27 bugs found.
scan-build: Run 'scan-view /builds/knot/knot-resolver/scan-results/2018-01-24-161139-14-1' to examine bug reports.
Uploading artifacts...
scan-results: found 32 matching files
Uploading artifacts to coordinator... ok
id=102160 responseStatus=201 Created token=Vz5xrD5a
ERROR: Job failed: exit code 1