Adds initial readme (#4)

#### 📖 Summary

- Adds initial readme

#### 📑 Test Plan

> 💡 Select your test plan for the code changes.

- [x] CI pipeline tests
- [ ] Custom test
- [ ] No test plan

##### Details / Justification

<!-- Add your test details or justification for missing tests here. -->

#### 📚 Additional Notes

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Co-authored-by: OCram85 <marco.blessing@googlemail.com>
Reviewed-on: #4

Dockerfile

@@ -23,6 +23,7 @@ ENV TINYPROXY_UID 5123
 ENV TINYPROXY_GID 5123
 
 ENV UPSTREAM_PROXY ""
+ENV UPSTREAM_PROXY_FILE ""
 ENV PORT "8888"
 ENV TIMEOUT "600"
 ENV LOGLEVEL "Info"
@@ -49,8 +50,6 @@ LogLevel $LOGLEVEL
 MaxClients $MAXCLIENTS
 ViaProxyName "tinyproxy"
 
-#upstream http $UPSTREAM_PROXY "."
-
 Filter "$FILTER_FILE"
 FilterURLs Off
 FilterCaseSensitive Off
@@ -58,8 +57,14 @@ FilterDefaultDeny Yes
 
 Allow 127.0.0.1/8
 Allow 10.0.0.0/8
+
 EOF
 
+RUN set -eu && \
+  CONFIG='/etc/tinyproxy/tinyproxy.conf' && \
+  [ -z "$UPSTREAM_PROXY_FILE" ] || export UPSTREAM_PROXY=$(cat $UPSTREAM_PROXY_FILE) && \
+  [ -z "$UPSTREAM_PROXY" ] || echo "upstream http $UPSTREAM_PROXY \".\"" >> "$CONFIG"
+
 RUN chown -R ${TINYPROXY_UID}:${TINYPROXY_GID} /etc/tinyproxy /var/log/tinyproxy
 USER ${TINYPROXY_UID}:${TINYPROXY_GID}