use slimmer image for init container

[sklarsa]

This PR swaps the data migration init container to a much smaller alpine distro to reduce the potential attack surface.

Since bash is not installed, we needed to update the migration script to make it POSIX-compliant.

[sklarsa]

I had claude run 3 tests locally using kind, and here are the results:

  Test 1: Fresh Install (test-fresh)                                                                               
                                                                                                                   
  Init container logs:                                                                                             
  File 'tables.d.*' not found. Nothing to move.                                                                    

  Pod status:
  NAME        READY   STATUS    RESTARTS   AGE   IP           NODE                       NOMINATED NODE   READINESS
   GATES
  questdb-0   1/1     Running   0          44s   10.244.0.6   test-fresh-control-plane   <none>           <none>

  ---
  Test 2: Old-to-New Migration (test-migrate)

  Seeded old data layout (PV root before migration):
  total 4
  drwxrwxrwx    1 root     root           142 Feb 18 16:20 .
  drwxr-xr-x    1 root     root            68 Feb 18 16:20 ..
  -rw-r--r--    1 root     root             0 Feb 18 16:20 .hidden_file
  -rw-r--r--    1 root     root             0 Feb 18 16:20 _tab_index.d
  -rw-r--r--    1 root     root             0 Feb 18 16:20 _txn
  drwxr-xr-x    1 root     root             0 Feb 18 16:20 conf
  -rw-r--r--    1 root     root             0 Feb 18 16:20 db.lock
  -rw-r--r--    1 root     root             9 Feb 18 16:20 somefile.txt
  -rw-r--r--    1 root     root             0 Feb 18 16:20 tables.d.0
  -rw-r--r--    1 root     root             0 Feb 18 16:20 tables.d.1
  SETUP-DONE

  Init container logs:
  Migration complete!

  /var/lib/questdb/ after migration (PV root — clean, only QuestDB-created dirs):
  drwxrwxrwx 1 questdb questdb  54 Feb 18 16:20 .
  drwxr-xr-x 1 root    root     48 Jan 28 15:24 ..
  drwxr-xr-x 1 questdb questdb  58 Feb 18 16:20 conf
  drwxr-xr-x 1 questdb questdb 770 Feb 18 16:20 db
  -rw-rw-r-- 1 questdb questdb 503 Feb 18 16:20 hello.txt
  drwxr-xr-x 1 questdb questdb  48 Feb 18 16:20 import
  drwxr-xr-x 1 questdb questdb  64 Feb 18 16:20 public

  /var/lib/questdb/db/ contents (all old files successfully moved in):
  -rw-r--r-- 1 questdb questdb    0 Feb 18 16:20 .hidden_file
  -rw-r--r-- 1 questdb questdb   16 Feb 18 16:20 .data_id
  drwxrwxr-x 1 questdb questdb   46 Feb 18 16:20 _query_trace
  -rw-r--r-- 1 questdb questdb 4096 Feb 18 16:20 _tab_index.d
  -rw-r--r-- 1 questdb questdb    0 Feb 18 16:20 _txn
  drwxr-xr-x 1 questdb questdb    0 Feb 18 16:20 conf
  -rw-r--r-- 1 questdb questdb    0 Feb 18 16:20 db.lock
  -rw-r--r-- 1 questdb questdb    9 Feb 18 16:20 somefile.txt
  -rw-r--r-- 1 questdb questdb    0 Feb 18 16:20 tables.d.0
  -rw-r--r-- 1 questdb questdb 4096 Feb 18 16:20 tables.d.1
  -rw-r--r-- 1 questdb questdb    0 Feb 18 16:20 tables.d.lock
  drwxrwxr-x 1 questdb questdb   66 Feb 18 16:20 telemetry
  ...

  ---
  Test 3: New-to-New Upgrade (test-upgrade)

  Old chart (debian+bash) installed, table created:
  {"ddl":"OK"}

  Upgraded to new chart (alpine+sh), init container logs:
  File 'tables.d.*' not found. Nothing to move.

  Table survived the upgrade:
  {"query":"SELECT * FROM test_table","columns":[{"name":"ts","type":"TIMESTAMP"},{"name":"val","type":"INT"}],"tim
  estamp":0,"dataset":[],"count":0}

[sklarsa]

Ran a few more tests...

  Test 4: Existing db/ directory — PASS                                                                            
                                                                                                                   
  - Seeded PV with tables.d.0, tables.d.1, AND an existing db/ dir containing existing_file.txt and                
  some_table/col.d                                                                                                 
  - Init log: "Migration complete!"                                                                                
  - Old db/ was moved to db/db/ (preserving existing_file.txt and some_table/ inside it)                           
  - All root files (rootfile.txt, _txn, .hidden, etc.) moved into db/                                              
  - QuestDB Running

  Test 5: Completely empty volume — PASS

  - PVC with zero files
  - Init log: "Nothing to move."
  - QuestDB started fine, created its own db/ directory
  - Pod 1/1 Running

  Test 6: Files with spaces and special characters — PASS

  - Seeded: file with spaces.txt, dir with spaces/inner.txt, quotefile.txt, .hidden_special
  - Init log: "Migration complete!"
  - All files moved into db/ with names intact: dir with spaces, file with spaces.txt, .hidden_special all present
  - QuestDB Running

[sklarsa]

More test results after code changes

  #: 1
  Scenario: Fresh install — empty PVC, no prior data
  Result: PASS
  Init Container Output: "Nothing to move."
  ────────────────────────────────────────
  #: 2
  Scenario: Old-to-new migration — seeded old layout with tables.d.*, hidden files
  Result: PASS
  Init Container Output: "Migration complete!" — all files including .hidden_file moved to db/
  ────────────────────────────────────────
  #: 3
  Scenario: New-to-new upgrade — install old chart (debian+bash), create table, upgrade to alpine+sh
  Result: PASS
  Init Container Output: "Nothing to move." — table survived upgrade
  ────────────────────────────────────────
  #: 4
  Scenario: Existing db/ directory — old layout + pre-existing db/ with files
  Result: PASS
  Init Container Output: "Migration complete!" — old db/ preserved at db/db/ with contents intact
  ────────────────────────────────────────
  #: 5
  Scenario: Empty volume — PVC with zero files
  Result: PASS
  Init Container Output: "Nothing to move." — QuestDB created its own db/
  ────────────────────────────────────────
  #: 6
  Scenario: Special characters — files with spaces, quotes, hidden dotfiles
  Result: PASS
  Init Container Output: "Migration complete!" — file with spaces.txt, dir with spaces, quote'file.txt, .hidden_special