ESP32: 2.0-r6
ESP8266: 2.0.0-1.5.0-r5

This brings updated mbedTLS with support for on-disk CA chains:
https://github.com/cesanta/mbedtls/compare/esp32_2.0-r5...esp32_2.0-r6
ESP8266 gets bigger rollup (ESP32 got those changes earlier, seems ok):
https://github.com/cesanta/mbedtls/compare/esp8266...esp8266_2.0.0-1.5.0-r5

Saves ~1.5K RAM for now, but will allow adding more roots to ca.pem without wasting RAM.

Refactored docker build for ESP8266 in the same way as was done earlier for ESP32.

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