Dockerfile: apt -> apt-get

Dockerfile commands should be using apt-get, not apt.
diff --git a/tests/docker/bionic/Dockerfile b/tests/docker/bionic/Dockerfile
index 1d96184..948e506 100644
--- a/tests/docker/bionic/Dockerfile
+++ b/tests/docker/bionic/Dockerfile
@@ -28,8 +28,8 @@
 
 ENV DEBIAN_FRONTEND noninteractive
 
-RUN apt update \
-    && apt -y install software-properties-common \
+RUN apt-get update \
+    && apt-get -y install software-properties-common \
     && rm -rf /var/lib/apt/lists
 
 RUN add-apt-repository -y ppa:team-gcc-arm-embedded/ppa