To list the contents of a docker named volume, run a temporary container and mount the volume into the container, then do a directory listing. Loop over all the volumes to see what each one holds.
~$ for i in `docker volume ls -q`; do echo "volume: ${i}"; docker run --rm -it -v ${i}:/vol alpine:latest ls /vol; echo; done;
volume: 140f898b1c69b85585942aa7f25cf03eba6ac66125d4a122e2fe99455c4a1a3f
volume: 1fa7b49173076a3a1fdb07ea7ce65d7187ff80e8b1a56e2fa667ebbbc0543f3a
dump.rdb
volume: 5564a11a1945567ffcc231145c01c806afe13a02b3e0a548f1504a1cd36c9374
dump.rdb
volume: 6d0b313416430d2abc0c872b98fd4180bbda4d14560c0a5d98f534f33b792164
app ib_buffer_pool private_key.pem
auto.cnf ib_logfile0 public_key.pem
ca-key.pem ib_logfile1 server-cert.pem
ca.pem ibdata1 server-key.pem
client-cert.pem mysql sys
client-key.pem performance_schema
volume: 8b08da4a38ca8f5924b90220db8c84384d90fe331a953a5aaa1a1944d826fc68
volume: 976239a3528b8b0b074b6b7438552e1d22c4f069cf20582d2250fcc1c068dc4f
dump.rdb
volume: aac262a93155286f4c551271d8d2a70f81ed1ca4cd56925e94006248e458895e
dump.rdb
volume: b385ebee063b72350fbc1158788cbb43d7da9b37ec95196b74caa6b22b1c115b
dump.rdb
volume: c73f2001f829e5574bd4246b2ab7a261a3f4d9a7ef89997765d7bf43883e5c24
dump.rdb
volume: ce73f9f85c475b1fd9cf4fede20fd04250ee7702e83db67c29c7118055275c28
dump.rdb
volume: foovolume1
volume: efc8a8855ac2c13d83c23573aebfd53e15072ec68d23e2793262f662ea0ae308
volume: foovolume2
auto.cnf ibdata1
ca-key.pem mysql
ca.pem performance_schema
client-cert.pem private_key.pem
client-key.pem public_key.pem
ib_buffer_pool server-key.pem
ib_logfile0 sys
ib_logfile1
volume: foovolume3
volume: phpsockettest
php-fpm.sock
volume: foovolume4
auto.cnf ib_logfile0 public_key.pem
ca-key.pem ib_logfile1 server-cert.pem
ca.pem ibdata1 server-key.pem
client-cert.pem mysql sys
client-key.pem performance_schema
ib_buffer_pool private_key.pem
testvol
asdf asdf1 asdf2
This installation has some test files, backing files from a few different mysql databases, a unix socket, redis files, and empty volumes.