build-wine: allow local testing

Before, it was only possible to test commits that are
on Github (pull request or merged). Now, changes can be
tested locally too.

This introduces the risk that a release could be built
containing uncommitted changes which by definition breaks
deterministic builds. Fortunately, this will always be
detected because the version string is created using
`git describe --tags --dirty`.

Also, retire $TARGET variable because it decouples the
build scripts from the commit revision to be built. This
is a problem for deterministic  builds.

contrib/build-wine/docker/README.md

@@ -25,14 +25,14 @@ folder.
 3. Build Windows binaries
 
     ```
-    $ TARGET=master
+    $ git checkout $REV
     $ sudo docker run \
         --name electrum-wine-builder-cont \
-        -v .:/opt/electrum \
+        -v $PWD:/opt/wine64/drive_c/electrum \
         --rm \
-        --workdir /opt/electrum/contrib/build-wine \
+        --workdir /opt/wine64/drive_c/electrum/contrib/build-wine \
         electrum-wine-builder-img \
-        ./build.sh $TARGET
+        ./build.sh
     ```
 4. The generated binaries are in `./contrib/build-wine/dist`.