Patching Home Assistant in Hass.io inside of Docker - where is the code?

Using the SSH/Terminal add-on package, I cannot locate the HA sources. Just wondering where I could access them in my HA system, to test a small patch? Thanks in advance.

Home Assistant 0.99.3 as Hass.io inside Docker on Debian 10 (Buster) on an Intel NUC i3.