[docker-ptf] create symlinks for py3-only image (sonic-net#19813)

Why I did it
The mix of docker-ptf with Python 2 + Python 3 (in virtual-env) and Python 3 images only across branches (with possibly some test scripts in sonic-mgmt not migrated to Python 3 in older branches) causes different path references to Python and PTF binaries. This can cause backporting a hassle. To prevent this the PR creates virtual environment (/root/env-python3) path to the real python 3 and PTF binaries. This keeps the paths same across docker-ptf images.

How I did it
Created symlinks for python and PTF from /root/env-python3/bin to /usr/bin.

How to verify it
Image built successfully and manually verified the existence of the links.

dockers/docker-ptf/Dockerfile.j2

@@ -18,6 +18,7 @@ LABEL maintainer="Pavel Shirshov"
 
 COPY ["sources.list.{{ CONFIGURED_ARCH }}", "/etc/apt/sources.list"]
 COPY ["no-check-valid-until", "/etc/apt/apt.conf.d"]
+COPY ["apt-retries-count", "/etc/apt/apt.conf.d"]
 
 ## Make apt-get non-interactive
 ENV DEBIAN_FRONTEND=noninteractive
@@ -266,6 +267,18 @@ RUN dpkg -i \
 debs/{{ deb }}{{' '}}
 {%- endfor %}
 
+{% if PTF_ENV_PY_VER == "py3" %}
+# Create symlink so that test scripts and ptf_runner invocation path
+# is same across python 2 and python 3 envs. Note that for virtual-env 
+# ptf is under /root/env-python3/bin. 
+# TODO - cleanup when the supported PTF image is py3only across all branches
+RUN mkdir -p /root/env-python3/bin \
+    && ln -s /usr/local/bin/ptf /usr/bin/ptf \
+    && ln -s /usr/bin/python /root/env-python3/bin/python3 \
+    && ln -s /usr/bin/python /root/env-python3/bin/python \
+    && ln -s /usr/local/bin/ptf /root/env-python3/bin/ptf
+{% endif %}
+
 COPY ["*.ini", "/etc/ptf/"]
 EXPOSE 22 8009