playbooks/upgrade-docker-ce: fix YAML literal-block indentation

Multi-line shell with backslash-continued URL had the continuation
line starting at column 0, which breaks YAML's | literal block
('could not find expected :'). Stash the URL into a shell variable
and emit on one logical line.

playbooks/upgrade-docker-ce.yaml

@@ -99,9 +99,9 @@ steps:
     shell: |
       codename=$(lsb_release -cs)
       arch=$(dpkg --print-architecture)
-      echo "deb [arch=${arch} signed-by=/etc/apt/keyrings/docker.gpg] \
-https://download.docker.com/linux/debian ${codename} stable" \
-        | sudo tee /etc/apt/sources.list.d/docker.list >/dev/null
+      url="https://download.docker.com/linux/debian"
+      line="deb [arch=${arch} signed-by=/etc/apt/keyrings/docker.gpg] ${url} ${codename} stable"
+      echo "${line}" | sudo tee /etc/apt/sources.list.d/docker.list >/dev/null
     sudo: true
     when: '[ ! -f /etc/apt/sources.list.d/docker.list ]'