build: try to consolidate instructions and decr codedupe in release.sh

contrib/build-linux/sdist/README.md

@@ -5,8 +5,7 @@ Source tarballs
    distributables that match the official releases._
 
 This assumes an Ubuntu (x86_64) host, but it should not be too hard to adapt to another
-similar system. The docker commands should be executed in the project's root
-folder.
+similar system.
 
 1. Install Docker
 
@@ -17,34 +16,14 @@ folder.
     $ sudo apt-get install -y docker-ce
     ```
 
-2. Build image
+2. Build source tarball
 
     ```
-    $ sudo docker build -t electrum-sdist-builder-img contrib/build-linux/sdist
+    $ ./build.sh
+    ```
+    If you want reproducibility, try instead e.g.:
+    ```
+    $ ELECBUILD_COMMIT=HEAD ELECBUILD_NOCACHE=1 ./build.sh
     ```
 
-3. Build source tarballs
-
-    It's recommended to build from a fresh clone
-    (but you can skip this if reproducibility is not necessary).
-
-    ```
-    $ FRESH_CLONE="contrib/build-linux/sdist/fresh_clone/electrum" && \
-        sudo rm -rf "$FRESH_CLONE" && \
-        umask 0022 && \
-        git clone . "$FRESH_CLONE" && \
-        cd "$FRESH_CLONE"
-    ```
-
-    And then build from this directory:
-    ```
-    $ git checkout $REV
-    $ sudo docker run -it \
-        --name electrum-sdist-builder-cont \
-        -v $PWD:/opt/electrum \
-        --rm \
-        --workdir /opt/electrum/contrib/build-linux/sdist \
-        electrum-sdist-builder-img \
-        ./make_sdist.sh
-    ```
-4. The generated distributables are in `./dist`.
+3. The generated distributables are in `./dist`.