Don't try using pip if custom server.

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "truss"
-version = "0.9.91"
+version = "0.9.92dev100"
 description = "A seamless bridge from model development to model delivery"
 license = "MIT"
 readme = "README.md"

truss/templates/base.Dockerfile.jinja

@@ -13,8 +13,10 @@ RUN $PYTHON_EXECUTABLE -c "import sys; \
     || { echo "ERROR: Supplied base image does not have {{ min_supported_python_version_in_custom_base_image }} <= python <= {{ max_supported_python_version_in_custom_base_image }}"; exit 1; }
 {% endblock %}
 
+{%- if not config.docker_server %}
 RUN pip install --upgrade pip --no-cache-dir \
     && rm -rf /root/.cache/pip
+{%- endif %}
 
 {% block base_image_patch %}
 {% endblock %}

truss/templates/server.Dockerfile.jinja

@@ -60,6 +60,7 @@ RUN mkdir -p {{ dst.parent }}; curl -L "{{ url }}" -o {{ dst }}
 {% endfor %} {#- endfor external_data_files #}
 {%- endif %} {#- endif external_data_files #}
 
+USER root
 {%- if build_commands %}
 {% for command in build_commands %}
 RUN {% for secret,path in config.build.secret_to_path_mapping.items() %} --mount=type=secret,id={{ secret }},target={{ path }}{%- endfor %} {{ command }}
@@ -134,6 +135,7 @@ RUN mkdir -p {{ supervisor_log_dir }}
 COPY supervisord.conf {{ supervisor_config_path }}
 ENV SUPERVISOR_SERVER_URL="{{ supervisor_server_url }}"
 ENV SERVER_START_CMD="supervisord -c {{ supervisor_config_path }}"
+
 ENTRYPOINT ["supervisord", "-c", "{{ supervisor_config_path }}"]
     {%- elif config.live_reload %} {#- elif config.live_reload #}
 ENV HASH_TRUSS="{{ truss_hash }}"