fix(docs): update deployment procedure for dockerized environment

Fixes: #4776

Changes:

- Switch to USER root when writing files under /etc, then switch
back to USER scylla for runtime.
Older Scylla images (e.g. 2025.1) used root as the default build
user, so writes to /etc succeeded. Newer images (2025.4) use non
root user scylla by default, causing Permission denied on /etc
writes.

- Replaced echo ... > file style writes with robust heredoc
writes. Heredocs avoid quoting/escaping issues and ensure valid
multiline config files.

- Removed version: "3.7" from compose file. Compose spec v2 no
longer requires it

Author: mikliapko

docs/source/docker/Dockerfile

@@ -2,16 +2,26 @@ FROM scylladb/scylla-manager-agent:latest as agent
 FROM scylladb/scylla:latest
 
 COPY --from=agent /usr/bin/scylla-manager-agent /usr/bin/
-RUN echo "[program:scylla-manager-agent]\n\
-command=/usr/bin/scylla-manager-agent\n\
-autorestart=true\n\
-stdout_logfile=/dev/stdout\n\
-stdout_logfile_maxbytes=0\n\
-stderr_logfile=/dev/stderr\n\
-stderr_logfile_maxbytes=0" > /etc/supervisord.conf.d/scylla-manager-agent.conf
-RUN mkdir -p /etc/scylla-manager-agent && echo "auth_token: token\n\
-s3:\n\
-    access_key_id: minio\n\
-    secret_access_key: minio123\n\
-    provider: Minio\n\
-    endpoint: http://minio:9000" > /etc/scylla-manager-agent/scylla-manager-agent.yaml
+
+USER root
+
+RUN cat > /etc/supervisord.conf.d/scylla-manager-agent.conf <<'EOF'
+[program:scylla-manager-agent]
+command=/usr/bin/scylla-manager-agent
+autorestart=true
+stdout_logfile=/dev/stdout
+stdout_logfile_maxbytes=0
+stderr_logfile=/dev/stderr
+stderr_logfile_maxbytes=0
+EOF
+
+RUN mkdir -p /etc/scylla-manager-agent && cat > /etc/scylla-manager-agent/scylla-manager-agent.yaml <<'EOF'
+auth_token: token
+s3:
+    access_key_id: minio
+    secret_access_key: minio123
+    provider: Minio
+    endpoint: http://minio:9000
+EOF
+
+USER scylla

docs/source/docker/docker-compose.yaml

@@ -1,5 +1,3 @@
-version: "3.7"
-
 services:
   scylla-manager:
     image: scylladb/scylla-manager