contrib: add docker_notes.md

contrib/android/Readme.md

@@ -13,12 +13,7 @@ similar system.
 
 1. Install Docker
 
-    ```
+    See `contrib/docker_notes.md`.
-    $ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
-    $ sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
-    $ sudo apt-get update
-    $ sudo apt-get install -y docker-ce
-    ```
 
 2. Build binaries
 

contrib/build-linux/appimage/README.md

@@ -14,12 +14,7 @@ see [issue #5159](https://github.com/spesmilo/electrum/issues/5159).
 
 1. Install Docker
 
-    ```
+    See `contrib/docker_notes.md`.
-    $ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
-    $ sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
-    $ sudo apt-get update
-    $ sudo apt-get install -y docker-ce
-    ```
 
 2. Build binary
 

contrib/build-linux/sdist/README.md

@@ -9,12 +9,7 @@ similar system.
 
 1. Install Docker
 
-    ```
+    See `contrib/docker_notes.md`.
-    $ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
-    $ sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
-    $ sudo apt-get update
-    $ sudo apt-get install -y docker-ce
-    ```
 
 2. Build source tarball
 

contrib/build-wine/README.md

@@ -9,12 +9,7 @@ similar system.
 
 1. Install Docker
 
-    ```
+    See `contrib/docker_notes.md`.
-    $ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
-    $ sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
-    $ sudo apt-get update
-    $ sudo apt-get install -y docker-ce
-    ```
 
     Note: older versions of Docker might not work well
     (see [#6971](https://github.com/spesmilo/electrum/issues/6971)).

contrib/docker_notes.md

@@ -0,0 +1,21 @@
+# Notes about using Docker in the build scripts
+
+- To install Docker:
+
+    This assumes an Ubuntu (x86_64) host, but it should not be too hard to adapt to another similar system.
+
+    ```
+    $ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
+    $ sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
+    $ sudo apt-get update
+    $ sudo apt-get install -y docker-ce
+    ```
+
+- To communicate with the docker daemon, the build scripts either need to be called via sudo,
+  or the unix user on the host system (e.g. the user you run as) needs to be
+  part of the `docker` group. i.e.:
+  ```
+  $ sudo usermod -aG docker ${USER}
+  ```
+  (and then reboot or similar for it to take effect)
+