How to set specs file for gcc in Docker container

I'm trying to build TREZOR firmware, it uses docker. Dockerfile contents:

RUN apt-get update
RUN apt-get install -y build-essential git python python-ecdsa gcc-arm-none-eabi

When gcc compiles libopencm3 in Docker, I've got the error:

error: 'for' loop initial declarations are only allowed in C99 mode note: use option -std=c99 or -std=gnu99 to compile your code

My first idea was to create specs file for gcc and add -std=c99 here. But as everything runs in the Docker container, I have no idea how to fix the problem. May be I wrong. Any suggestions? Thanks