Automate detection of proxy variables for android build

**Context**

Unity is pretty stupid when it comes to building for android behind a proxy (without any access to the internet without the proxy). See also this thread:

https://forum.unity.com/threads/request-proxy-settings-for-unity-hub.515447/

One needs to edit the sdkmanager script file directly in order to make it work, no environment variable setting works for passing those proxy variables into the sdkmanager that is called during the android build.

Android builds behind a proxy _eventually_ succeed but it takes roughly 6 times longer (60 minutes vs 11) with all the extra time spent waiting for connection timeouts.

**Suggested solution**

Add the following to the Dockerfile for Unity:

```
find /opt/unity -name "sdkmanager" -exec sed -i 's/APP_ARGS"/APP_ARGS" ${SDKMANAGER_PROXY}/' {} \;
```

Also add something to the init files for android that does:
```
if [ -z ${PROXY_HOST+x} ]; then
export SDKMANAGER_PROXY="--proxy=http --proxy_host="\$PROXY_HOST" --proxy_port="\$PROXY_PORT"
fi
```

If you'd like I can provide a pull request that does both of these things.

**Considered alternatives**

Manually doing that, which is what we are doing now.