After I found this SO entry, I aplied chmod a+rw /proc as suggested.
Now, watchdogand ofelia are running / starting but I see this error followed by a stack trace while starting the dovecot container:
# docker compose up -d
[+] Running 13/15
✔ Container mailcowdockerized-memcached-mailcow-1 Running 0.0s
✔ Container mailcowdockerized-sogo-mailcow-1 Running 0.0s
✔ Container mailcowdockerized-dockerapi-mailcow-1 Running 0.0s
✔ Container mailcowdockerized-olefy-mailcow-1 Running 0.0s
✔ Container mailcowdockerized-redis-mailcow-1 Running 0.0s
✔ Container mailcowdockerized-php-fpm-mailcow-1 Running 0.0s
⠿ Container mailcowdockerized-nginx-mailcow-1 Starting 45.2s
✔ Container mailcowdockerized-acme-mailcow-1 Running 0.0s
✔ Container mailcowdockerized-watchdog-mailcow-1 Running 0.0s
✔ Container mailcowdockerized-unbound-mailcow-1 Running 0.0s
✔ Container mailcowdockerized-mysql-mailcow-1 Running 0.0s
✔ Container mailcowdockerized-solr-mailcow-1 Running 0.0s
✔ Container mailcowdockerized-clamd-mailcow-1 Running 0.0s
✔ Container mailcowdockerized-postfix-mailcow-1 Running 0.0s
⠴ Container mailcowdockerized-dovecot-mailcow-1 Starting 45.1s
Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: error during container init: error running hook #0: error ru
nning hook: exit status 2, stdout: , stderr: runtime/cgo: pthread_create failed: Resource temporarily unavailable
SIGABRT: abort
PC=0x7f6a759c1a7c m=4 sigcode=18446744073709551610
goroutine 0 [idle]:
runtime: g 0: unknown pc 0x7f6a759c1a7c
stack: frame={sp:0x7f6a4d12d610, fp:0x0} stack=[0x7f6a4c92e060,0x7f6a4d12dc60)
0x00007f6a4d12d510: 0x0000000000000000 0x0000000000000000
0x00007f6a4d12d520: 0x0000000000000000 0x0000000000000000
0x00007f6a4d12d530: 0x0000000000000000 0x0000000000000000
0x00007f6a4d12d540: 0x00007f6a4d12d5c8 0x00007f6a4d12dbc8
0x00007f6a4d12d550: 0x0000559461de8119 <runtime.netpoll+0x00000000000000d9> 0x0000000000000000
0x00007f6a4d12d560: 0x00007f6a4d12d5c8 0x0000000000000000