Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/external/vanetza-idf/vanetza/asn1/Dockerfile

@@ -0,0 +1,18 @@
+FROM ubuntu:latest AS base
+
+FROM base AS build
+RUN apt-get update && apt-get install -y autoconf automake bison build-essential flex git libtool
+ARG ASN1C_REPO=https://github.com/mouse07410/asn1c.git
+ARG ASN1C_VERSION=eefc4cfca40e5ee6e0b6506d6d8c93c24d423cec
+RUN git clone $ASN1C_REPO /asn1c-source && cd /asn1c-source && git checkout ${ASN1C_VERSION}
+WORKDIR /asn1c-source
+RUN autoreconf -iv && ./configure --prefix /asn1c && make -j $(nproc) install
+
+FROM build AS test
+RUN make -j $(nproc) check
+RUN make -j $(nproc) distcheck
+
+FROM base AS runtime
+RUN apt-get update && apt-get install -y cmake
+COPY --from=build /asn1c/bin /usr/bin
+COPY --from=build /asn1c/share /usr/share