Dockerfile: rm dpkg --add-architecture lines

Dockerfile used to install libseccomp-dev packages for different
architectures. This is no longer true since commit f30244ee1b, which
changed to cross-compiling libseccomp (so we can get a static library
to link against).

Thus, adding extra architectures is no longer needed.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Dockerfile

@@ -9,10 +9,6 @@ ARG CRIU_REPO=https://download.opensuse.org/repositories/devel:/tools:/criu/Debi
 RUN KEYFILE=/usr/share/keyrings/criu-repo-keyring.gpg; \
     wget -nv $CRIU_REPO/Release.key -O- | gpg --dearmor > "$KEYFILE" \
     && echo "deb [signed-by=$KEYFILE] $CRIU_REPO/ /" > /etc/apt/sources.list.d/criu.list \
-    && dpkg --add-architecture armel \
-    && dpkg --add-architecture armhf \
-    && dpkg --add-architecture arm64 \
-    && dpkg --add-architecture ppc64el \
     && apt-get update \
     && apt-get install -y --no-install-recommends \
         build-essential \