Simplify dockerfile and move a few things back to Jenkinsfile

Bruce makes the argument that the Dockerfile should only contain things
that need to be done as root.

I don't have a strong opinion on the topic so I am accommodating this.

Dockerfile.CI → .ci/Dockerfile

@@ -21,20 +21,9 @@
 FROM ubuntu:24.04
 
 WORKDIR /tmp/spead2
-COPY . .
+COPY .ci/install-sys-pkgs.sh .
 
 # Install dependencies
 # jq and iproute2 are used to find the interface IPv4 address
 RUN apt-get update && apt-get -y --no-install-recommends install libpython3-dev python3-venv jq iproute2
-RUN python3 -m venv /venv
 RUN .ci/install-sys-pkgs.sh
-RUN PATH="/venv/bin:$PATH" .ci/py-requirements.sh
-
-# Install Python package
-RUN PATH="/venv/bin:$PATH" pip install -v \
-    --config-settings=setup-args=--native-file=ci.ini \
-    --config-settings=setup-args=-Dibv=enabled \
-    --config-settings=setup-args=-Dibv_hw_rate_limit=enabled \
-    --config-settings=setup-args=-Dmlx5dv=enabled \
-    .
-

Jenkinsfile

@@ -25,7 +25,7 @@ pipeline {
   agent {
     dockerfile {
       label 'spead2'
-      filename 'Dockerfile.CI'
+      dir '.ci'
       registryCredentialsId 'dockerhub'  // Supply credentials to avoid rate limit
       // These arguments allow direct access to the NVIDIA NIC with ibverbs
       args '--network=host --ulimit=memlock=-1 -e NVIDIA_MOFED=enabled -e NVIDIA_VISIBLE_DEVICES=all --runtime=nvidia'
@@ -42,7 +42,23 @@ pipeline {
   }
 
   stages {
+    stage('Install dependencies') {
+      steps {
+        sh 'python3 -m venv /venv'
+        sh 'PATH="/venv/bin:$PATH" py-requirements.sh'
 
+      }
+    }
+    stage('Install Python package') {
+      steps {
+        sh '''PATH="/venv/bin:$PATH" pip install -v \
+          --config-settings=setup-args=--native-file=ci.ini \
+          --config-settings=setup-args=-Dibv=enabled \
+          --config-settings=setup-args=-Dibv_hw_rate_limit=enabled \
+          --config-settings=setup-args=-Dmlx5dv=enabled \
+          .'''
+      }
+    }
     stage('Run tests') {
       steps {
         sh 'PATH="/venv/bin:$PATH" .ci/py-tests-jenkins.sh'