When running the following pipeline an unexpected exception is raised

pipeline {
    agent { label "docker"}
    environment {
        DOCKER_HOST="tcp://192.168.1.209:2375"
    }
    stages {
        stage('Back-end') {
            agent {
                docker { image 'testdocker.uex-repo-test.hq.k.grp/nagra/cd/centos-5-python-jenkinsagent:latest' }
            }
            steps {
                sh "ls -la"
          }
        }
    }
}

The exception raised is the following:
Started by user
Teichner Peter
[Pipeline] nodeRunning on
build-docker-01
in /home/jenkins/workspace/CD/PipelineDocker-Test[Pipeline] {[Pipeline] withEnv[Pipeline] {[Pipeline] stage[Pipeline] { (Back-end)[Pipeline] nodeRunning on
Jenkins
in /var/lib/jenkins/jobs/CD/jobs/PipelineDocker-Test/workspace[Pipeline] {[Pipeline] withEnv[Pipeline] {[Pipeline] withDockerRegistry[Pipeline] // withDockerRegistry[Pipeline] }[Pipeline] // withEnv[Pipeline] }[Pipeline] // node[Pipeline] }[Pipeline] // stage[Pipeline] }[Pipeline] // withEnv[Pipeline] }[Pipeline] // node[Pipeline] End of Pipelinejava.net.MalformedURLException: no protocol: testdocker.uex-repo-test.hq.k.grp
at java.net.URL.<init>(URL.java:593)
at java.net.URL.<init>(URL.java:490)
at java.net.URL.<init>(URL.java:439)
at org.jenkinsci.plugins.docker.commons.credentials.DockerRegistryEndpoint.getEffectiveUrl(DockerRegistryEndpoint.java:136)
at org.jenkinsci.plugins.docker.commons.credentials.DockerRegistryEndpoint.newKeyMaterialFactory(DockerRegistryEndpoint.java:219)
at org.jenkinsci.plugins.docker.workflow.RegistryEndpointStep$Execution.newKeyMaterialFactory(RegistryEndpointStep.java:65)
at org.jenkinsci.plugins.docker.workflow.AbstractEndpointStepExecution.start(AbstractEndpointStepExecution.java:44)
at org.jenkinsci.plugins.workflow.cps.DSL.invokeStep(DSL.java:224)
at org.jenkinsci.plugins.workflow.cps.DSL.invokeMethod(DSL.java:150)
at org.jenkinsci.plugins.workflow.cps.CpsScript.invokeMethod(CpsScript.java:108)
at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.call(PogoMetaClassSite.java:48)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
at com.cloudbees.groovy.cps.sandbox.DefaultInvoker.methodCall(DefaultInvoker.java:19)
at org.jenkinsci.plugins.docker.workflow.Docker.withRegistry(jar:file:/var/lib/jenkins/plugins/docker-workflow/WEB-INF/lib/docker-workflow.jar!/org/jenkinsci/plugins/docker/workflow/Docker.groovy:37)
at __cps.transform__(Native Method)
at com.cloudbees.groovy.cps.impl.ContinuationGroup.methodCall(ContinuationGroup.java:57)
at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.dispatchOrArg(FunctionCallBlock.java:109)
at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.fixArg(FunctionCallBlock.java:82)
at sun.reflect.GeneratedMethodAccessor341.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.cloudbees.groovy.cps.impl.ContinuationPtr$ContinuationImpl.receive(ContinuationPtr.java:72)
at com.cloudbees.groovy.cps.impl.ClosureBlock.eval(ClosureBlock.java:46)
at com.cloudbees.groovy.cps.Next.step(Next.java:83)
at com.cloudbees.groovy.cps.Continuable$1.call(Continuable.java:174)
at com.cloudbees.groovy.cps.Continuable$1.call(Continuable.java:163)
at org.codehaus.groovy.runtime.GroovyCategorySupport$ThreadCategoryInfo.use(GroovyCategorySupport.java:122)
at org.codehaus.groovy.runtime.GroovyCategorySupport.use(GroovyCategorySupport.java:261)
at com.cloudbees.groovy.cps.Continuable.run0(Continuable.java:163)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.access$001(SandboxContinuable.java:19)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable$1.call(SandboxContinuable.java:35)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable$1.call(SandboxContinuable.java:32)
at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.GroovySandbox.runInSandbox(GroovySandbox.java:108)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.run0(SandboxContinuable.java:32)
at org.jenkinsci.plugins.workflow.cps.CpsThread.runNextChunk(CpsThread.java:174)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.run(CpsThreadGroup.java:330)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.access$100(CpsThreadGroup.java:82)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:242)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:230)
at org.jenkinsci.plugins.workflow.cps.CpsVmExecutorService$2.call(CpsVmExecutorService.java:64)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at hudson.remoting.SingleLaneExecutorService$1.run(SingleLaneExecutorService.java:112)
at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:748)
Finished: FAILURE