wine build: pin wine signing key. minor refactoring.

contrib/build-wine/docker/Dockerfile

@@ -9,19 +9,10 @@ RUN dpkg --add-architecture i386 && \
         gnupg2=2.2.4-1ubuntu1.2 \
         dirmngr=2.2.4-1ubuntu1.2 \
         python3-software-properties=0.96.24.32.1 \
-        software-properties-common=0.96.24.32.1 \
+        software-properties-common=0.96.24.32.1
-        && \
+
-    wget -nc https://dl.winehq.org/wine-builds/Release.key && \
+RUN apt-get update -q && \
-    wget -nc https://dl.winehq.org/wine-builds/winehq.key && \
+        apt-get install -qy \
-    apt-key add Release.key && \
-    apt-key add winehq.key && \
-    apt-add-repository https://dl.winehq.org/wine-builds/ubuntu/ && \
-    apt-get update -q && \
-    apt-get install -qy \
-        wine-stable-amd64:amd64=3.0.1~bionic \
-        wine-stable-i386:i386=3.0.1~bionic \
-        wine-stable:amd64=3.0.1~bionic \
-        winehq-stable:amd64=3.0.1~bionic \
         git=1:2.17.1-1ubuntu0.4 \
         p7zip-full=16.02+dfsg-6 \
         make=4.1-9.1ubuntu1 \
@@ -29,8 +20,22 @@ RUN dpkg --add-architecture i386 && \
         autotools-dev=20180224.1 \
         autoconf=2.69-11 \
         libtool=2.4.6-2 \
-        gettext=0.19.8.1-6 \
+        gettext=0.19.8.1-6
-        && \
+
-    rm -rf /var/lib/apt/lists/* && \
+RUN wget -nc https://dl.winehq.org/wine-builds/Release.key && \
+        echo "c51bcb8cc4a12abfbd7c7660eaf90f49674d15e222c262f27e6c96429111b822 Release.key" | sha256sum -c - && \
+        apt-key add Release.key && \
+    wget -nc https://dl.winehq.org/wine-builds/winehq.key && \
+        echo "78b185fabdb323971d13bd329fefc8038e08559aa51c4996de18db0639a51df6 winehq.key" | sha256sum -c - && \
+        apt-key add winehq.key && \
+    apt-add-repository https://dl.winehq.org/wine-builds/ubuntu/ && \
+    apt-get update -q && \
+    apt-get install -qy \
+        wine-stable-amd64:amd64=3.0.1~bionic \
+        wine-stable-i386:i386=3.0.1~bionic \
+        wine-stable:amd64=3.0.1~bionic \
+        winehq-stable:amd64=3.0.1~bionic
+
+RUN rm -rf /var/lib/apt/lists/* && \
     apt-get autoremove -y && \
     apt-get clean

contrib/build-wine/docker/README.md

@@ -20,7 +20,7 @@ folder.
 2. Build image
 
     ```
-    $ sudo docker build --no-cache -t electrum-wine-builder-img contrib/build-wine/docker
+    $ sudo docker build -t electrum-wine-builder-img contrib/build-wine/docker
     ```
 
     Note: see [this](https://stackoverflow.com/a/40516974/7499128) if having dns problems