build: simplify detection of the target platform

It can be done within the container, rather than having to compute it in
the Makefile and then pass it as an argument.

jibri/Dockerfile

@@ -8,7 +8,6 @@ LABEL org.opencontainers.image.url="https://github.com/jitsi/jibri"
 LABEL org.opencontainers.image.source="https://github.com/jitsi/docker-jitsi-meet"
 LABEL org.opencontainers.image.documentation="https://jitsi.github.io/handbook/"
 
-ARG TARGETPLATFORM
 ARG USE_CHROMIUM=0
 #ARG CHROME_RELEASE=latest
 # https://googlechromelabs.github.io/chrome-for-testing/
@@ -21,9 +20,11 @@ RUN apt-dpkg-wrap apt-get update && \
     /usr/bin/install-chrome.sh && \
     apt-cleanup && \
     adduser jibri rtkit && \
-    case ${TARGETPLATFORM} in \
-        "linux/amd64")  SC_ARCH=x86_64  ;; \
-        "linux/arm64")  SC_ARCH=aarch64  ;; \
+    dpkgArch="$(dpkg --print-architecture)" && \
+    case "${dpkgArch##*-}" in \
+        "amd64") SC_ARCH=x86_64 ;; \
+        "arm64") SC_ARCH=aarch64 ;; \
+        *) echo "unsupported architecture"; exit 1 ;; \
     esac && \
     wget -qO /usr/bin/shm-check https://github.com/saghul/shm-check/releases/download/v1.0.0/shm-check-${SC_ARCH} && \
     chmod +x /usr/bin/shm-check

jibri/rootfs/usr/bin/install-chrome.sh

@@ -2,7 +2,9 @@
 
 set -o pipefail -xeu
 
-if [ "${USE_CHROMIUM}" = 1 -o "${TARGETPLATFORM}" = "linux/arm64" ]; then
+dpkgArch="$(dpkg --print-architecture)"
+
+if [ "${USE_CHROMIUM}" = 1 -o "${dpkgArch##*-}" = "arm64" ]; then
     echo "Using Debian's Chromium"
     apt-dpkg-wrap apt-get install -y chromium chromium-driver chromium-sandbox
     chromium --version