Unable to run rootless with custom directories

Description of Issue

Trying to set up a container image running salt-master rootless (see setup below for Dockerfile), and while I specify all the directories (see master config below) I get the following error:

Failed to create directory path "/var/cache/salt/master/queues" - [Errno 13] Permission denied: '/var/cache/salt'

In the master config I've specified:

cachedir: /tmp/salt/cache

This seems to be ignored.
Yes, I'm aware of the root_dir option, I'm trying to define a custom directory structure.

Setup

Dockerfile (minimal reference):

FROM alpine:3
RUN apk add --no-cache salt-master && \
    adduser -DU -u 100 foo && \
    mkdir /data /config && \
    chown 100:100 /data /config
COPY --chown=100:100 /tmp/master /config/master
USER 100:100
CMD ["/usr/bin/salt-master", "--config-dir=/config"]

/tmp/master:

pki_dir: /data/pki
cachedir: /tmp/salt/cache
pidfile: /tmp/salt/master.pid
sock_dir: /tmp/salt/master
log_file: /data/logs/master
conf_file: /config/master

file_roots:
  base:
    - /data/states
pillar_roots:
  base:
    - /data/pillars

Steps to Reproduce Issue

See above files then run (as a normal user):

podman build -t saltmaster:latest .
podman run --rm saltmaster:latest

Versions Report

Salt Version:
           Salt: 2019.2.2

Dependency Versions:
           cffi: 1.13.2
       cherrypy: Not Installed
       dateutil: 2.8.1
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.10.3
        libgit2: 0.28.4
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.6.2
   mysql-python: Not Installed
      pycparser: 2.19
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: 1.0.0
         Python: 3.8.1 (default, Dec 30 2019, 15:43:37)
   python-gnupg: Not Installed
         PyYAML: 5.2
          PyZMQ: 18.1.1
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 6.0.3
            ZMQ: 4.3.2

System Versions:
           dist: alpine 3.11.3
         locale: utf-8
        machine: x86_64
        release: 5.4.10-200.fc31.x86_64
         system: Linux
        version: Alpine Linux 3.11.3

Start with the provided Dockerfile and /tmp/master configuration, then reproduce the failure with the podman build and podman run commands. Trace why salt-master uses /var/cache/salt/master/queues instead of the configured cachedir; done means the rootless container starts with the custom directory settings without a permission error.