ci: update dockerfiles to support different knot versions

.gitlab-ci.yml

-image: $CI_REGISTRY/knot/knot-resolver/ci:debian-stable
+image: $CI_REGISTRY/knot/knot-resolver/ci/debian-stable:knot-2.6
 
 variables:
   DEBIAN_FRONTEND: noninteractive
@@ -32,7 +32,7 @@ build:linux:amd64:
 
 build:clang:linux:amd64:
   stage: build
-  image: $CI_REGISTRY/knot/knot-resolver/ci:debian-unstable  # newer Debian for newer Clang
+  image: $CI_REGISTRY/knot/knot-resolver/ci/debian-unstable:knot-2.6  # newer Debian for newer Clang
   script:
     - rm daemon/lua/kres-gen.lua
     - CXX=clang++-5.0 CC=clang-5.0 make -k all "CFLAGS=-Werror -Wno-error=unused-command-line-argument -ggdb"
@@ -70,7 +70,7 @@ lint:lua:
 
 lint:c:
   stage: test
-  image: $CI_REGISTRY/knot/knot-resolver/ci:debian-unstable  # newer Debian for newer Clang
+  image: $CI_REGISTRY/knot/knot-resolver/ci/debian-unstable:knot-2.6  # newer Debian for newer Clang
   dependencies: []  # do not download build artifacts
   script:
     - make lint-c CLANG_TIDY="clang-tidy-5.0 -quiet"
@@ -80,7 +80,7 @@ lint:c:
 lint:clang-scan-build:
   allow_failure: true  # for now it is just informative
   stage: test
-  image: $CI_REGISTRY/knot/knot-resolver/ci:debian-unstable  # newer Debian for newer Clang
+  image: $CI_REGISTRY/knot/knot-resolver/ci/debian-unstable:knot-2.6  # newer Debian for newer Clang
   dependencies: []  # do not download build artifacts
   script:
     - MAKEFLAGS="-k -j$(nproc)" SCAN_BUILD="/usr/lib/llvm-5.0/bin/scan-build -o scan-results --status-bugs -no-failure-reports" ./tests/clang_scan_build.sh make

ci/README.md

+Docker Build
+------------
+
+* debian-stable / debian-unstable
+
+```
+$ export KNOT_BRANCH=2.6
+$ docker build -t registry.labs.nic.cz/knot/knot-resolver/ci/debian-stable:knot-$KNOT_BRANCH --build-arg KNOT_BRANCH=$KNOT_BRANCH debian-stable
+$ docker build -t registry.labs.nic.cz/knot/knot-resolver/ci/debian-unstable:knot-$KNOT_BRANCH --build-arg KNOT_BRANCH=$KNOT_BRANCH debian-unstable
+
+$ docker login registry.labs.nic.cz
+$ docker push registry.labs.nic.cz/knot/knot-resolver/ci/debian-stable:knot-$KNOT_BRANCH
+$ docker push registry.labs.nic.cz/knot/knot-resolver/ci/debian-unstable:knot-$KNOT_BRANCH
+```

ci/debian-stable/Dockerfile

 FROM debian:stable
 MAINTAINER Knot Resolver <knot-resolver@labs.nic.cz>
+ARG KNOT_BRANCH=master
 
 WORKDIR /root
 CMD ["/bin/bash"]
@@ -33,11 +34,11 @@ RUN pip3 install -r /tmp/deckard-req.txt
 
 # build and install latest version of Knot DNS
 # (kresd depends on libknot and libdnssec)
-RUN git clone --depth=1 --branch=2.6 https://gitlab.labs.nic.cz/knot/knot-dns.git /tmp/knot
+RUN git clone --depth=1 --branch=$KNOT_BRANCH https://gitlab.labs.nic.cz/knot/knot-dns.git /tmp/knot
 WORKDIR /tmp/knot
 RUN pwd
 RUN autoreconf -if
-RUN ./configure
+RUN ./configure --prefix=/usr
 RUN make
 RUN make install
 RUN ldconfig

ci/debian-unstable/Dockerfile

 FROM debian:unstable
 MAINTAINER Knot Resolver <knot-resolver@labs.nic.cz>
+ARG KNOT_BRANCH=master
 
 WORKDIR /root
 CMD ["/bin/bash"]
@@ -13,11 +14,11 @@ RUN apt-get install -y -qqq make cmake pkg-config git build-essential bsdmainuti
 
 # build and install latest version of Knot DNS
 # (kresd depends on libknot and libdnssec)
-RUN git clone --depth=1 --branch=2.6 https://gitlab.labs.nic.cz/knot/knot-dns.git /tmp/knot
+RUN git clone --depth=1 --branch=$KNOT_BRANCH https://gitlab.labs.nic.cz/knot/knot-dns.git /tmp/knot
 WORKDIR /tmp/knot
 RUN pwd
 RUN autoreconf -if
-RUN ./configure
+RUN ./configure --prefix=/usr
 RUN make
 RUN make install
 RUN ldconfig