Dockerfile: Relocate and rework oe-git-proxy installation

Moving this file to /usr/local/bin broke user configurations that had
/usr/bin/ hard-coded, and it also broke inside the kas sandbox which
does not have /usr/local/bin in its PATH. So move things back to the
original location.

Do that by setting a link to the /kas/contrib folder, rather than
copying the file once more.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Dockerfile

@@ -26,14 +26,14 @@ RUN apt-get install --no-install-recommends -y \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
-COPY contrib/oe-git-proxy /usr/local/bin/
-ENV GIT_PROXY_COMMAND="oe-git-proxy" \
-    NO_PROXY="*"
-
 COPY . /kas
 RUN pip3 --proxy=$https_proxy install --no-deps kconfiglib && \
     pip3 --proxy=$https_proxy install --no-deps /kas && kas --help
 
+RUN ln -s /kas/contrib/oe-git-proxy /usr/bin/
+ENV GIT_PROXY_COMMAND="oe-git-proxy" \
+    NO_PROXY="*"
+
 RUN echo "builder ALL=NOPASSWD: ALL" > /etc/sudoers.d/builder-nopasswd && \
     chmod 660 /etc/sudoers.d/builder-nopasswd