common.dockcross: Split minimal "dockcross" from 'debian' and 'docker'

Makefile

@@ -22,7 +22,7 @@ GEN_IMAGE_DOCKERFILES = $(addsuffix /Dockerfile,$(GEN_IMAGES))
 # These images are expected to have explicit rules for *both* build and testing
 NON_STANDARD_IMAGES = web-wasm manylinux1-x64 manylinux1-x86 manylinux2010-x64 manylinux2010-x86 manylinux2014-x64 manylinux2014-aarch64
 
-DOCKER_COMPOSITE_SOURCES = common.docker common.debian common.manylinux common.crosstool common.windows common-manylinux.crosstool
+DOCKER_COMPOSITE_SOURCES = common.docker common.debian common.manylinux common.crosstool common.windows common-manylinux.crosstool common.dockcross
 
 # This list all available images
 IMAGES = $(STANDARD_IMAGES) $(NON_STANDARD_IMAGES)
@@ -68,6 +68,7 @@ $(GEN_IMAGE_DOCKERFILES) Dockerfile: %Dockerfile: %Dockerfile.in $(DOCKER_COMPOS
 		-e '/common.crosstool/ r common.crosstool' \
 		-e '/common-manylinux.crosstool/ r common-manylinux.crosstool' \
 		-e '/common.windows/ r common.windows' \
+		-e '/common.dockcross/ r common.dockcross' \
 		$< > $@
 
 #

common.dockcross

@@ -0,0 +1,26 @@
+WORKDIR /usr/src
+
+# Image build scripts
+COPY \
+  imagefiles/install-gosu-binary.sh \
+  imagefiles/install-gosu-binary-wrapper.sh \
+  /buildscripts/
+
+RUN \
+  apt-get update --yes && \
+  apt-get install --no-install-recommends --yes \
+    curl \
+    dirmngr \
+    gpg \
+  && \
+  apt-get clean --yes && \
+  /buildscripts/install-gosu-binary.sh && \
+  /buildscripts/install-gosu-binary-wrapper.sh && \
+  rm -rf /buildscripts
+
+RUN echo "root:root" | chpasswd
+WORKDIR /work
+ENTRYPOINT ["/dockcross/entrypoint.sh"]
+
+# Runtime scripts
+COPY imagefiles/entrypoint.sh imagefiles/dockcross /dockcross/