Rename Dockerfile to Dockerfile.in to ensure consistency

All Dockerfiles are now configured similarly, this will support
future improvements.

Makefile

@@ -16,7 +16,7 @@ BIN = ./bin
 STANDARD_IMAGES = linux-s390x android-arm android-arm64 android-x86 android-x86_64 linux-x86 linux-x64 linux-arm64 linux-arm64-musl linux-armv5 linux-armv5-musl linux-armv6 linux-armv6-musl linux-armv7 linux-armv7a linux-armv7l-musl linux-mips linux-mipsel linux-ppc64le windows-static-x86 windows-static-x64 windows-static-x64-posix windows-shared-x86 windows-shared-x64 windows-shared-x64-posix
 
 # Generated Dockerfiles.
-GEN_IMAGES = linux-s390x linux-mips manylinux1-x64 manylinux1-x86 manylinux2010-x64 manylinux2010-x86 manylinux2014-x64 manylinux2014-x86 manylinux2014-aarch64 web-wasm linux-arm64 linux-arm64-musl windows-static-x86 windows-static-x64 windows-static-x64-posix windows-shared-x86 windows-shared-x64 windows-shared-x64-posix linux-armv7 linux-armv7a linux-armv7l-musl linux-armv6-musl linux-armv5 linux-armv5-musl linux-ppc64le
+GEN_IMAGES = linux-s390x android-arm android-arm64 linux-x86 linux-x64 linux-mips linux-mipsel manylinux1-x64 manylinux1-x86 manylinux2010-x64 manylinux2010-x86 manylinux2014-x64 manylinux2014-x86 manylinux2014-aarch64 web-wasm linux-arm64 linux-arm64-musl windows-static-x86 windows-static-x64 windows-static-x64-posix windows-shared-x86 windows-shared-x64 windows-shared-x64-posix linux-armv7 linux-armv7a linux-armv7l-musl linux-armv6 linux-armv6-musl linux-armv5 linux-armv5-musl linux-ppc64le
 GEN_IMAGE_DOCKERFILES = $(addsuffix /Dockerfile,$(GEN_IMAGES))
 
 # These images are expected to have explicit rules for *both* build and testing