tools/docker: Add libpython2.7 in order to satisfy GDB dependencies

It was not possible to run xtensa-esp32-elf-gdb from container due to
missing libpython2.7 library.

Merges https://github.com/espressif/esp-idf/pull/5817
Closes https://github.com/espressif/esp-idf/issues/5284

tools/docker/Dockerfile

@@ -2,6 +2,7 @@ FROM ubuntu:18.04
 
 ARG DEBIAN_FRONTEND=noninteractive
 
+# We need libpython2.7 due to GDB tools
 RUN apt-get update && apt-get install -y \
     apt-utils \
     bison \
@@ -17,6 +18,7 @@ RUN apt-get update && apt-get install -y \
     libusb-1.0-0-dev \
     make \
     ninja-build \
+    libpython2.7 \
     python3 \
     python3-pip \
     unzip \