feat(synadm): Switch image build process to requirements.txt

This patch tries to improve reproduciblity by using a
`requirements.txt`, this should not only pin a version, but
also allow renovate to update it automatically on a regular
basis, making updates more visible.

images/synadm/.release

-release=0.1.0
+release=0.2.0

images/synadm/Dockerfile

 FROM docker.io/library/python:3
 
+COPY requirements.txt .
+
 RUN true \
-  && pip3 install synadm \
+  && pip3 install -r requirements.txt \
   && mkdir -p /workspace \
   && true
 

images/synadm/requirements.txt

+synadm==0.37.1