Don't prefix $FATCAT_VERSION with "v"

The fatcat release tarballs don't follow a common pattern.

e.g there is:
- https://github.com/Gregwar/fatcat/archive/v1.1.0.tar.gz (v prefix)
- https://github.com/Gregwar/fatcat/archive/1.0.7.tar.gz (no v prefix)

The "v" prefix can be passed in as $FATCAT_VERSION when it's needed.

This allows for both tarball formats to be supported with --build-arg FATCAT_VERSION=1.0.7 and --build-arg FATCAT_VERSION=v1.1.0

Dockerfile

@@ -37,8 +37,8 @@ RUN strip "arm-softmmu/qemu-system-arm" "aarch64-softmmu/qemu-system-aarch64"
 
 # Build stage for fatcat
 FROM debian:stable-slim AS fatcat-builder
-ARG FATCAT_VERSION=1.1.0
+ARG FATCAT_VERSION=v1.1.0
-ENV FATCAT_TARBALL="v${FATCAT_VERSION}.tar.gz"
+ENV FATCAT_TARBALL="${FATCAT_VERSION}.tar.gz"
 WORKDIR /fatcat
 
 RUN # Update package lists
@@ -53,7 +53,7 @@ RUN tar xvf "${FATCAT_TARBALL}"
 
 RUN # Build source
 RUN apt-get -y install build-essential cmake
-RUN cmake "fatcat-${FATCAT_VERSION}" -DCMAKE_CXX_FLAGS='-static'
+RUN cmake fatcat-* -DCMAKE_CXX_FLAGS='-static'
 RUN make -j$(nproc)