Docker + VirtualBox VM + Alpine - FS issue

2-console-output.log

$ pwd
/home/luis/code/_sandbox/testing-alpine

$ echo 'foo' > file.txt

$ ls
file.txt

$ cat file.txt
foo

$ sha256sum file.txt
b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c  file.txt

$ docker run -it --rm -v $(pwd):/app --workdir /app alpine:3.6 cat file.txt
foo

$ docker run -it --rm -v $(pwd):/app --workdir /app alpine:3.6 sha256sum file.txt
b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c  file.txt

$ echo 'bar' >> file.txt

$ cat file.txt
foo
bar

$ sha256sum file.txt
d78931fcf2660108eec0d6674ecb4e02401b5256a6b5ee82527766ef6d198c67  file.txt

$ docker run -it --rm -v $(pwd):/app --workdir /app alpine:3.6 sha256sum file.txt
d78931fcf2660108eec0d6674ecb4e02401b5256a6b5ee82527766ef6d198c67  file.txt

$ docker run -it --rm -v $(pwd):/app --workdir /app alpine:3.6 cat file.txt
foo

$ hexdump -C file.txt
00000000  66 6f 6f 0a 62 61 72 0a                           |foo.bar.|
00000008

$ docker run -it --rm -v $(pwd):/app --workdir /app alpine:3.6 hexdump -C file.txt
00000000  66 6f 6f 0a 00 00 00 00                           |foo.....|
00000008

3-new-file-output.log

$ echo 'foo' > file2.txt

$ hexdump -C file2.txt
00000000  66 6f 6f 0a                                       |foo.|
00000004

$ docker run -it --rm -v $(pwd):/app --workdir /app alpine:3.6 hexdump -C file2.txt
00000000  66 6f 6f 0a                                       |foo.|
00000004

4-linux-output.log

$ pwd
/home/luis/code/_sandbox/test-alpine

$ uname -a
Linux Seyori 4.10.0-35-generic #39-Ubuntu SMP Wed Sep 13 07:46:59 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

$ echo 'foo' > file3.txt

$ hexdump -C file3.txt 
00000000  66 6f 6f 0a                                       |foo.|
00000004

$ docker run -it --rm -v $(pwd):/app --workdir /app alpine:3.6 hexdump -C file3.txt
00000000  66 6f 6f 0a                                       |foo.|
00000004

$ echo 'bar' >> file3.txt

$ hexdump -C file3.txt 
00000000  66 6f 6f 0a 62 61 72 0a                           |foo.bar.|
00000008

$ docker run -it --rm -v $(pwd):/app --workdir /app alpine:3.6 hexdump -C file3.txt
00000000  66 6f 6f 0a 00 00 00 00                           |foo.....|
00000008