Restore custom image for Cross

Cross.toml

@@ -1,5 +1,5 @@
 [target.x86_64-unknown-linux-gnu]
-pre-build = ["apt-get install -y cmake clang-3.9"]
+dockerfile = './scripts/cross/Dockerfile'
 
 [target.aarch64-unknown-linux-gnu]
-pre-build = ["apt-get install -y cmake clang-3.9"]
+dockerfile = './scripts/cross/Dockerfile'

scripts/cross/Dockerfile

@@ -0,0 +1,25 @@
+ARG CROSS_BASE_IMAGE
+FROM $CROSS_BASE_IMAGE
+
+RUN apt-get update -y && apt-get upgrade -y
+
+RUN apt-get install -y unzip && \
+    PB_REL="https://github.com/protocolbuffers/protobuf/releases" && \
+    curl -L $PB_REL/download/v3.15.8/protoc-3.15.8-linux-x86_64.zip -o protoc.zip && \
+    unzip protoc.zip -d /usr && \
+    chmod +x /usr/bin/protoc
+
+RUN apt-get install -y \
+    apt-transport-https \
+    ca-certificates \
+    gnupg-agent \
+    software-properties-common
+
+RUN curl -L https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add - && \
+    apt-add-repository "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-5.0 main" && \
+    apt-get update && \
+    apt-get install -y clang-5.0
+
+RUN apt-get install -y cmake
+
+ENV PROTOC=/usr/bin/protoc