tests-extra: update dependencies, remove outdated Docker instructions

tests-extra/Dockerfile

-FROM cznic/knot:latest
-MAINTAINER Marek Vavrusa <marek.vavrusa@nic.cz>
-
-# Environment
-ENV RUNTIME_PKGS gawk python3 python3-pip bind9 bind9utils valgrind lsof gdb
-
-# Copy test cases
-COPY . /knot-src/tests-extra
-
-# Select entrypoint
-WORKDIR /knot-src/tests-extra
-ENTRYPOINT ["/knot-src/tests-extra/runtests.py"]
-
-# Install dependencies and sources
-RUN apt-get -q -y update && \
-apt-get install -q -y ${RUNTIME_PKGS} && \
-pip3 install -r /knot-src/tests-extra/requirements.txt && \
-# Link the prebuilt executables
-mkdir -p /knot-src/src && \
-ln -sfn /usr/local/sbin/knotd /knot-src/src/knotd && \
-ln -sfn /usr/local/sbin/knotc /knot-src/src/knotc && \
-# Trim down the image
-apt-get autoremove -q -y && \
-apt-get clean && \
-rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*

tests-extra/README

 Prerequisites:
-==============
+--------------
 
 Python >=3.3
 dnspython >=1.16.0
@@ -7,7 +7,7 @@ python3-psutil
 dnssec-signzone
 dnssec-keygen
 dnssec-verify
-Bind 9.9.x
+Bind >= 9.11
 lsof
 gawk
 objdump
@@ -36,22 +36,3 @@ Allow ptrace:
 or
 # vim /etc/sysctl.d/10-ptrace.conf
 # kernel.yama.ptrace_scope = 0
-
-Running in docker container:
-----------------------------
-
-There is a Dockerfile that can be used to build containers with the tests-extra,
-the main benefit is that each test runs in an isolated container and can be run in parallel.
-
-Run tests on the upstream branch:
----------------------------------
-
-$ docker pull cznic/knot:tests-extra # Or build with 'docker build -t tests-extra .'
-$ docker run -it -v cznic/knot:tests-extra basic
-
-Run tests on the local executables:
------------------------------------
-
-You can bind-mount the /src with the current working copy.
-
-$ docker run -it -v $(pwd)/..:/knot-src cznic/knot:tests-extra basic

tests-extra/requirements.txt

-git+https://github.com/rthalley/dnspython.git@4f26a2c7af86474a3800c616aa26f2f419dd28c5#egg=dnspython
-#dnspython>=1.16.0
+dnspython>=1.16.0
 psutil