You can do using an init script that you can call at docker entry point.

I don't think it is possible right now, without a 3rd party tool. regarding api ref it does not support multi variable in YAML. But I will tell you about a 3rd party tool -- Helm

It is possible to achieve it using Helm. your template will look like:

 containers:
    - name: {{.Values.Backend.name }}
      image: "{{ .Values.Backend.image.repository }}:{{ .Values.Backend.image.tag }}"
      imagePullPolicy: "{{ .Values.Backend.image.pullPolicy }}"
      args:
        - name: TARGET_URL
          value: {{ .Value.URL}}
        - name: TARGET_PARAM
          value: {{ .Value.PARAM}}
        - name: URL
          value: {{ .Value.URL }}/mysite/{{ .Value.PARAM}}

and you will add to the file values.yaml parameters for TARGET_URL and TARGET_PARAM

URL: http://www.example.com
PARAM: param          

For environment variables (and a couple of other fields in the pod spec, including args and command) there is a Make-like $(VARIABLE) name that will get expanded; see for example the documentation for .env.value. This could look like:

env:
  - name: TARGET_URL
    valueFrom:
      configMapKeyRef:
        key: cm
        name: TARGET_URL
  - name: TARGET_PARAM
    valueFrom:
      configMapKeyRef:
        key: cm
        name: TARGET_PARAM
  - name: TARGET_DETAIL_URL
    value: $(TARGET_URL)/mysite/$(TARGET_PARAM)

If you are depending on mounting a ConfigMap into a container as files, then it can only contain static content; this trick won't work.