Inline Docker build script

With #6193, both Docker variants are built on different machines, so
that not both binaries need to be handled at once.

Also, the linker flags shouldn't be necessary with binaries being
linked statically on Linux by default.

Author: SimplyDanny

Dockerfile

@@ -10,9 +10,9 @@ COPY Plugins Plugins/
 COPY Source Source/
 COPY Tests Tests/
 COPY Package.* ./
-COPY tools/build-linux-release.sh tools/
 ARG TARGETPLATFORM
-RUN --mount=type=cache,target=/workspace/.build,id=build-$TARGETPLATFORM ./tools/build-linux-release.sh
+RUN swift build -c release --product swiftlint
+RUN mv $(swift build -c release --show-bin-path)/swiftlint .
 
 # Runtime image
 FROM ubuntu:${UBUNTU_VERSION} AS runtime
@@ -50,9 +50,7 @@ COPY --from=builder /usr/lib/swift/linux/libswiftDispatch.so /usr/lib
 COPY --from=builder /usr/lib/swift/linux/libswiftGlibc.so /usr/lib
 COPY --from=builder /usr/lib/swift/linux/libswiftSynchronization.so /usr/lib
 COPY --from=builder /usr/lib/swift/linux/libswiftSwiftOnoneSupport.so /usr/lib
-COPY --from=builder /workspace/swiftlint_linux_* /usr/bin
-
-RUN ln -s /usr/bin/swiftlint_linux_* /usr/bin/swiftlint
+COPY --from=builder /workspace/swiftlint /usr/bin
 
 RUN swiftlint version
 RUN echo "_ = 0" | swiftlint --use-stdin