build: LTO profile 'thin' and moved to Cargo.toml

It makes more sense to set the release LTO profile into the related Cargo.toml section than into the Dockerfile only.

Using LTO 'thin' profile because according to the documentation it is a good compromise with no or little performance loss.

docker/Dockerfile

@@ -21,8 +21,7 @@ RUN test -x build/duniter || \
 ARG threads=1
 RUN test -x build/duniter || \
     ( \
-        CARGO_PROFILE_RELEASE_LTO="true" \
-            cargo build --release -j $threads && \
+        cargo build --release -j $threads && \
         mkdir -p build && \
         mv target/release/duniter build/ \
     )