Docker-本地图形验证码测试正常，使用docker上线后报空指针问题

问题描述：

最近搭建一个项目，本地测试正常。使用docker做server服务器。图形验证码代码逻辑报空指针异常。

java.lang.NullPointerException
    at sun.awt.FontConfiguration.getVersion(FontConfiguration.java:1264)
    at sun.awt.FontConfiguration.readFontConfigFile(FontConfiguration.java:219)
    at sun.awt.FontConfiguration.init(FontConfiguration.java:107)
    at sun.awt.X11FontManager.createFontConfiguration(X11FontManager.java:774)
    at sun.font.SunFontManager$2.run(SunFontManager.java:431)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.font.SunFontManager.<init>(SunFontManager.java:376)
    at sun.awt.FcFontManager.<init>(FcFontManager.java:35)
    at sun.awt.X11FontManager.<init>(X11FontManager.java:57)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at java.lang.Class.newInstance(Class.java:442)
    at sun.font.FontManagerFactory$1.run(FontManagerFactory.java:83)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.font.FontManagerFactory.getInstance(FontManagerFactory.java:74)
    at java.awt.Font.getFont2D(Font.java:491)
    at java.awt.Font.access$000(Font.java:224)
    at java.awt.Font$FontAccessImpl.getFont2D(Font.java:228)
    at sun.font.FontUtilities.getFont2D(FontUtilities.java:180)
    at sun.java2d.SunGraphics2D.checkFontInfo(SunGraphics2D.java:670)
    at sun.java2d.SunGraphics2D.getFontInfo(SunGraphics2D.java:831)
    at sun.java2d.pipe.GlyphListPipe.drawString(GlyphListPipe.java:50)
    at sun.java2d.SunGraphics2D.drawString(SunGraphics2D.java:2926)
    at com.miaoshaproject.util.CodeUtil.generateCodeAndPic(CodeUtil.java:74)

这个问题让我怀疑人生好几天。最后发现是jdk的锅。 开始的Dockerfile使用的是
FROM openjdk:8-jdk-alpine
最后发现，openjdk镜像中对于一些jdk底层jar包并不支持，后来使用
FROM frolvlad/alpine-oraclejdk8:slim 镜像完美解决问题。
发现问题，一定要敢于质疑一切。哪怕是jdk！